Age of rootstock tree for grafting
Please can you send some photos of the old tree.
In theory you can graft onto old trees, but it depends on the health of the tree.
I know the technique as "top-working". You can either bark graft or shoot graft.
It does require a fair amount of work to be successful. How skilled are you in grafting and tree management? Is this a home garden or small or large orchard?
It is a home garden with two apricot trees of approximately similar age. The photo I have attached is from June 2016, but not a good shot as it was not intended as a specimen. Can't do another for you now (pouring outside!) and can only give you one when I get back home in early January. The person who told us we can do it wants to prune the tree back and wait for new shoots to grow, onto which he says he will graft.
The tree looks healthy, I'm sure a good grafter will be able to be successful topworking this tree.
There are some goods videos on YouTube (https://www.youtube.com/watch?v=Yty_MfVYa5c https://www.youtube.com/watch?v=qTtXmBVsolY) showing bark grafting.
Remember to leave a small "breather" branch to keep the sap moving. This can be removed when the grafted wood is a decent size. And paint the tree white the same day you remove the branches to prevent sunburn.