Thanks for your question about replacing your trellis foy your concord grapes.  I have included 2 articles that I think will give you a good start.  Since your vines are so old, I could not tell if you have been pruning them.  Both articles include good info about pruning and trellis construction.  Each article has more info than I copied, but I have given you enough to see if it is helpful.  If you need additional info, please feel free to contact me.                                                                                          https://extension.usu.edu/yardandgarden/research/grape-trellising-training-basics

How to Grow Grapes: Trellising and Training Basics

Grapes are a wonderful addition to the home garden. With proper care and pruning they are longlived and productive plants. This fact sheet discusses basic pruning information and introduces two common training methods suitable for the Utah home garden. There are many different types of training systems used for grape production throughout the world. The best system for your grape vine depends on the cultivar grown, the climate, soil fertility, and personal preference. This fact sheet details two common systems, the 4-cane single-trunk Kniffin system (cane pruned) and bilateral high cordon (spur pruned). Both systems work well for Utah home growers. Other grape systems can be used successfully, but for simplicity only these two systems are discussed here.

Planning is one of the most important steps for successfully growing grapes. It can be tempting to simply buy a grape vine and plant it somewhere in the yard with visions of delicious grapes in your future. However, without attention to detail your grape vine may not be very fruitful or long lasting. First, familiarize yourself with what is needed for grape vine management as well as planting basics. The companion fact sheet Grape Vine Management is a helpful resource for understanding what is needed for success. Next, select the grape variety you would like to grow, paying attention to your climate as well as your desired use of the fruit. See the companion fact sheet Grape Varieties for Utah for a list of varieties that can be grown in Utah. Once you know what type of grape you will be planting and have selected a suitable place on your property, it is time to consider your trellising system and how you will train your grape vine.

Trellis Construction

For most training systems, grape vines must have support. This can be in the form of a trellis, arbor, or possibly a fence. However, it is important to consider the challenges of pruning when selecting the type of support to use. Chain-link or similar styled fence can present unnecessary challenges as the canes become intertwined in the fencing material making them particularly difficult to remove during pruning. There are many ways to trellis grapes. It is best to choose and construct your trellis before planting. Grape trellises are typically either single posts in a line connected by one to four lines of wire (Fig. 1) or posts with a cross bar at the top with two to four lines of wire running between the horizontal cross bars.

Trellis posts should be treated wood (metal posts will also work, but wire attachment is more difficult) and need to be long enough to be firmly anchored and still leave around 6 feet above the ground. Place inline posts about 20 to 25 feet apart. Use heavy galvanized wire (No. 9 or 10 gauge, high tensile) to support the weight of the vine. The number of wires you use will vary by the training system. Pound fencing staples into the windward side of the posts at the desired heights, leaving a space large enough to allow the wire to pass through. Thread the wire through the staples, making sure the wire can still move side to side. Using a turnbuckle or similar device to attach the wire to the post at one end can allow for adjusting the tension of the wire. See Fig. 1 for an example trellis system. Braces are used at the end of the trellis to provide support and anchorage. Note in this example that end braces are constructed by placing the last two posts closer together with a cross bar between them and twisted wire running diagonally to reinforce the brace. If more than one row is desired, leave 10 to 12 feet between rows to facilitate harvest and maintenance. The distance between plants in a row depends on variety and trellising method, and can range from 6 to 12 feet.

Terminology

A basic understanding of grape vine structure and terminology is helpful for understanding pruning and training. The appropriately maintained grape vine consists of both permanent and seasonal wood. The trunk, also called the stem, is permanent wood. A cordon is semi-permanent wood that is trained horizontally along the trellis wire. Succulent new growth arising from a bud on older wood is called a shoot. After the shoot drops its leaves in the fall it is called a cane. A spur is a cane that has been pruned back to one to five buds. The top of the trunk where the vine transitions to cordon, spurs, or canes is called the head. On each shoot or cane there are buds that form at the base of each leaf that will produce the leaves, shoots, and flowers the following season.

Pruning

Fruiting shoots grow from 1-year old dormant buds. Pruning is done to replace the fruiting wood each year. Grape pruning requires removal of between 80 and 90% of the canes each year. A fully mature, healthy vine can support a total maximum of 40 shoots. Younger vines will need to have fewer shoots until they reach maturity. Limiting the number of shoots is important for good cluster size, root balance, and preparation for winter. Grape pruning is done in one of two ways depending on the variety and training system: cane pruning or spur pruning. In general, American type grapes and many hybrids should be cane pruned because their basal buds are typically only vegetative (leaves but no fruit). This means if you are spur pruning them you would be cutting off the buds that are most likely to produce fruit. European grapes can be cane or spur pruned. If you are not sure which pruning method would be better for your variety, it is best to use cane pruning. Each of the two methods are described below.

  https://growing-concord-grapes.blogspot.com/2012/05/concord-grape-trellis-and-training.html

Concord Grape Trellis and Training Systems

Grapevines can be grown to conform to numerous shapes. Arbors, fences, and decorative trellises are only a few of the possibilities. Since many home gardeners may opt for less traditional training systems, a few general principles should be kept in mind, when pruning and training your grapevines. As with any fruit crops, this primary pruning should be done in the dormant season.

1. The grapevines should be trained to reasonably fill the structure but not become overgrown. This is easier said than done, since initially the vine grows fairly slowly, but as it matures, it may become a monster of vegetation. One to two layers of leaves for any area on the canopy is best for flower bud and fruit development.



2. Mature grapevines, by their nature, produce much more wood than they can support. Think of the wild grapevine growing in the forest: it produces a huge amount of wood just to climb up to the sunlight. Your garden grapevine will not need to do that, but they still produce much more wood than is necessary or desirable. Typically, during dormant pruning you need to remove as much as 90 percent of the new growth on a mature grapevine. Plan on leaving about three or four buds per foot of cordon (the horizontal trunk on a grapevine) length. Dormant pruning is usually done in late winter or early spring.


3. Grapes bear their fruit on one-year-old wood.

4. Different grape cultivars have different growth habits. The canes of American cultivars tend to grow in a willowy, downward direction, whereas those of the European cultivars and many French-American hybrids tend to grow directly up. Choose your training system with this in mind.

5. Grapevines vary considerably in their vigor, due to both genetic and environmental factors. Because of this variation, it is difficult to make exact recommendations as to how many buds, or how large a crop, to leave on the vine in any given year. Vigorous vines can support and ripen a larger load of fruit than less vigorous vines. In other words, you can leave more buds for next year's fruit development on a big and robust vine than you can on a little waif of a thing. Therefore, you need to a judgment about how many buds to leave during pruning based on how much growth the plant achieved in the previous growing season, which can be estimated by the amount of wood you have to remove during dormant pruning. As a general rule, plants that are relatively weak growing should have two buds per foot of cordon (row) or 30 buds per plant retained, whereas vigorous vines should have 3 to 4 buds per foot of cordon or 40 to 45 buds per plants retained. This concept is called "balanced pruning" because it balances the crop for next season with last season's growth.

Remember that although our methods of pruning and training are based on science and experimentation, much of grape pruning relies on experience and, for lack of a better word, art. These instructions seem a little befuddling at first glance, but as you engage in the process of pruning, you will understand them more fully and come to appreciate the time spent in your vineyard in winter, in sweet anticipation of the summer to come.

By way of guidance, some traditional training systems employed by commercial and backyard growers are described here. All the various trellis systems refer to a vine in the early spring after dormant pruning.

High-Wire Cordon

American cultivars such as 'Concord' or 'Niagara' tend to produce shoots that grow in a downward direction, so it generally makes sense to put the permanent horizontal branch on a high wire and let the plant drape toward the ground. In the high-wire cordon system, also known as the Hudson River Umbrella, a single horizontal wire is secured at a height of 6 feet, and another is placed at a 3-foot height. The lower wire will be used to hold the vine up during early training, and later, after the plant is trained, it will only be used to secure the trunk in place.

In training a vine to this system, select two strong canes and train them up to the top wire; then, as the canes grow, place them along the top wire, in opposite directions. Be careful not to wrap the canes around the wire, since they will expand in girth over time and can girdle themselves on the wire. It's better to tie the canes to the top wire at first. They will eventually send out tendrils to secure themselves to the wire, but they will need a little help initially. Make sure that you keep an eye on the string that is securing the canes to the wire, since the string can also girdle the canes as they grow. Cordons from one vine should not overlap with those of adjacent vines. Ideally, you would like to train up the first two canes in the first year of growth, but realistically it often takes two years to get the cordons trained to the upper wire. Have patience; they will eventually get there, provided they are in the appropriate climate. As in the case with growing many other perennial small- fruit crops your training and patience in the first years will pay off in future decades.

Once the cordons are established, they should be pruned so that you are left with several shoots or spurs that are five to seven buds long, for a total of 30 to 45 buds per plant, depending on the vigor of the plant, as discussed earlier. These spurs should be spaced 6 to 12 inches apart on the cordon. In addition, for each seven-bud spur, leave one two-bud renewal spur. The smaller spurs will provide the shoots for the following year.

The fruiting shoots will hang like a curtain in groups from the spurs that originate from the arms along the top wire. Shoots should be carefully separated and placed vertically downward from the top wire for a distance of 18 to 24 inches. Positioning should be carried out as soon as the shoots have toughened, usually 2 to 3 weeks after peak bloom (when 50 percent of the fused petals have fallen). Extreme care must be exercised during shoot positioning, as any shoot lost at this time can result in a poorly filled trellis.

In subsequent years, select new canes and leave new renewal spurs from the shoots that developed from the previous year's buds. Remember to balance the number of buds with the vigor of the plant: a high number of buds should be left on vigorous plants than on weaker ones.

Low-Wire Cordon

The idea behind the low- wire, or low-bilateral, cordon training system is similar to that of the Hudson River system just described. The low-wire system, however, is used for grapevines that have a tendency to grow up rather than down, as is the case with the European grapes and most of the French-American hybrids. If you put these forms on a Hudson River system, the shoots would grow up from the top, 6-foot high wire- not a good thing. The low-wire cordon system takes advantage of the plant's natural tendency to grow up, allowing the shoots from the permanent cordons on the lower, 3-foot high wire to fill in the trellis from the bottom up. Of course, gravity tends to work against you, so as the shoots grow up, you need to help them by using catch wires that sandwich the new shoots between them as they grow up. Usually, two sets of catch wires are adequate.

Umbrella Kniffen System

In the Umbrella Kniffen system, the plant's trunk is brought up and tied to a 6-foot high wire, with four or more canes left near the top of the trunk bearing a total of 50 to 60 buds. All other wood except for two renewal spurs is removed from near the head. After pruning to the desired number of buds, retie the trunk to the top wire. The canes should be bent rather sharply over the top wire so that the other outer bark cracks, and the tips are tied to the lower, 3-foot high wire. The renewal buds will develop into shoots that probably will not be fruitful but should be allowed to grow. They are there to be used for next year's fruiting canes. The buds on the main canes will form fruiting shoots that do not need to be tied because the vine has already been trained. Some of those shoots may be well located and can replace the original canes the following season, in which case the renewal shoots are not needed.

Four-Cane Kniffen System

The Four-cane Kniffen system is a variation on the Umbrella Kniffen system in which the canes are selected from both the top and the middle of the trunk. It is often used on low- vigor cultivars. The lower canes are shaded by the upper canes, and the lower ones often produce later-maturing fruit that may be lower in quantity and/or quality than that on the cordons trained on the top wire.

Other Training Systems

The production of 'Thompson Seedless' grapes in California and similar climates requires cane-pruning systems, since the buds closest to the base are not fruitful, yet those are the only buds that are left on the vine in cordon systems. If you trained 'Thompson Seedless' to a low -wire cordon, you would get a lot of leaves and no fruit. The most commonly employed training system for this cultivar is head training. Head training involves tying the main trunk to a vertical post.

Another common training system is the Geneva Double Curtain. This system is good for extremely vigorous vines since it allows the plant to achieve its potential yield in a relatively small space. Weaker growing vines would not fill such a large trellis system.

 growth into the selected trunk. Loosely tie the selected shoot to a vertical stake that has been secured to the trellis wire. Once it grows a few inches past the lower trellis wire, pinch back the shoot tip to induce branching, leaving two buds above the wire. Several shoots near the top of the cane will develop. Select three shoots to keep and loosely secure the lower two horizontally to the wire, growing in opposite directions (see Fig. 4), with the third positioned vertically toward the second wire. This new growth will need to be secured to the horizontal wires every couple of weeks throughout the growing season. Remove all other shoots originating from the trunk as they appear. Timing of pinching back the vertical trunk at the second wire depends on when the branch reaches the top wire. If the vertical shoot reaches the top trellis wire late in the first growing season, wait to top the shoot until February or March in the dormant season. If the shoot does not grow to the top wire by the dormant season, continue training the cane to the top trellis wire in the second season. If any fruit clusters develop during the establishment year(s), remove them to allow the grape plant to develop a good root system.

Cane Establishment

Once the trunk is established and the four selected canes have been secured to the horizontal wires, the first dormant cane pruning should take place. Prune at the end of February or the beginning of March. On the bottom two canes, count out from the trunk to four or six buds and prune off the remainder of the cane. On the top two canes, leave only three to five buds on each cane. During the following growing season, depending on the variety and vigor of the grape vine, flowers may develop. At this point, you can allow them to develop into fruit, but if more than one flower cluster forms per shoot, remove the bottom cluster before bloom. Do not do any summer pruning. During the second dormant season during cane establishment, keep one shoot from the base of each cane as new fruiting wood and prune one shoot back to two to three buds for a renewal spur (Fig. 2). Renewal spurs will grow the canes for next year’s production (Fig. 3). Tip each fruiting cane back to 10 or fewer buds, depending on the vigor of the plant. Canes differ in fruitfulness. When selecting which canes to keep, look for ones that are pencil width in diameter, grew in the light, and do not have abnormally long spaces between buds.

Fully Established

The renewal spurs left in the previous year will have grown canes from the two buds that were left. Choose the better of the two canes and prune it back to 10 buds to keep as next year’s fruiting cane. Prune the other cane back to a two-bud renewal spur. Remove last year’s fruiting cane. If the renewal spur did not produce a satisfactory replacement cane, one of the base shoots on last year’s fruiting cane can be selected instead. Remember, a fully mature, healthy grape plant should be able to support a maximum total of 40 buds. A classic mistake of many homeowners is to leave more buds than this because removing so much wood each year is intimidating. Stick to the plan! Leaving more than 40 buds will limit the plant’s ability to build up food reserves and increases the chance of winter injury. It also reduces the quality of the fruit produced.

C

Fruit Specialist

Plants, Soils and Climate

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