Once collards bolt, they don't truly revert back to leafy production. You might be able to coax a small second flush of leaves from the side shoots for a little while. Collards are cool-season crops. A stretch of warm days signals the plant to switch from leaf production to reproduction. Once this shift occurs, the plant's priority shifts to seeds, not leaves. Those tiny leaves you're seeing in the "crotches" (leaf axils) are side shoots, and yes, they can produce new leaves; however, the growth will be smaller, the leaves may be bitter, and the plant will most likely bolt again quickly. You may get a couple more weeks of usable leaves, but this plant is nearing the end of its productive cycle. You can switch to a more heat-tolerant green, like Malabar spinach or Swiss chard, to handle our summer heat. 

Here is the UnMD Fact Sheet on growing greens. The warm weather caused your collards to bolt. 

You can replant for a fall harvest.

Flower Stalks Form or Bolting of Vegetables | University of Maryland Extension

I also did an internet search and found the following which I reviewed and think the info is correct.

 Sorry for the format, but I could not change it.

Bolting collard greens can still be used, but their leaves may become bitter; removing flower stalks and adjusting care can prolong leaf harvest.

Understanding Bolting

Collard greens bolt when environmental stress, such as extreme heat, cold, or inconsistent watering, triggers the plant to produce flowers and seeds instead of leaves. Once bolting begins, the leaves often become tougher, more fibrous, and develop a bitter flavor because the plant diverts energy to flower and seed production. Bolting is a natural survival mechanism, but it can reduce the quality of the leaves for fresh consumption.Green Packs+2

Managing Bolting Plants

• Remove Flower Stalks: Snip off emerging flower stalks promptly to redirect the plant’s energy back into leaf production, which can temporarily slow bolting. 2
• Regular Harvesting: Frequently harvesting leaves encourages new growth and delays the plant’s transition to flowering. 1
• Watering and Mulching: Keep soil consistently moist to reduce stress-induced bolting. Mulching can help regulate soil temperature and retain moisture. 2
• Shade and Temperature Control: Provide shade during hot spells or use row covers to protect plants from temperature extremes. 2
• Select Bolt-Resistant Varieties: Cultivars like ‘Georgia Southern’ or ‘Flash’ are more resistant to bolting and can extend the harvest period. 2
• 4 Sources

Using Bolted Collard Greens

Even if leaves are bitter, bolted collard greens can still be used in cooked dishes:

• Braising or Steaming: Cooking helps tenderize tougher leaves and reduces bitterness. 1
• Soups and Stews: Bolted greens add flavor and nutrients when incorporated into hearty dishes. 1
• Reviving Wilted Leaves: Soak leaves in cold water or ice water for 10–30 minutes to restore some crispness before cooking. 1
• 1 Source

Preventing Future Bolting

• Plant collards in the appropriate season, avoiding extreme temperatures. 2
• Ensure soil is fertile, well-drained, and rich in nutrients like nitrogen, phosphorus, and potassium. 1
• Rotate crops annually to reduce stress and disease pressure. 1
  By combining careful pruning, proper watering, and timely harvesting, you can extend the productive life of your collard greens and make the most of bolting plants in the kitchen.
• 2 Sources

Thank you.

Thank you, very helpful. I assume it is too late to buy new collard plants to replace the bolted ones? I have 6 Tuscan kale plants doing well. (I cannot eat chard though I love it.)

Given the prediction for cool weather in May, you can try to plant some new seedlings but the chances of a good yield are not great. You do not have time to plant from seeds. I would wait until fall.

This came from another Expert who recommended to plant Malabar spinach, and next year look for a more heat tolerant collard variety:

"Once collards bolt, they don't truly revert back to leafy production. You might be able to coax a small second flush of leaves from the side shoots for a little while. Collards are cool-season crops. A stretch of warm days signals the plant to switch from leaf production to reproduction. Once this shift occurs, the plant's priority shifts to seeds, not leaves. Those tiny leaves you're seeing in the "crotches" (leaf axils) are side shoots, and yes, they can produce new leaves; however, the growth will be smaller, the leaves may be bitter, and the plant will most likely bolt again quickly. You may get a couple more weeks of usable leaves, but this plant is nearing the end of its productive cycle. You can switch to a more heat-tolerant green, like Malabar spinach or Swiss chard, to handle our summer heat."