A few years ago, I bought an orchard that was very heterogeneous. Most trees bloomed early in winter and their fruit set was poor. Fruit yield was not very good for many of them. That orchard was doomed not to be profitable, so an intervention was necesary.
In 2015, I detected 2 bitter almond trees in that orchard. I chopped them down one metre from the surface in autumn 2015. Many shots appeared in spring 2016. I selected 3 to 4 shots in mid May and grafted them. The technique I use is ring budding, as explained in the post about grafting almond trees (https://almondorchard.wordpress.com/2015/08/11/generating-trees-from-seeds/). New shots of the good variety had grown by mid June. The shots from the rootstock are carefully eliminated as they appear. Since the rootstock is healthy and the tree is strong, the shots grow very quickly and the tree may yield sweet almonds in two years.
I have rejuvenated the whole orchard using this technique in 2017. It has been hard work, but I consider it more sensible tan replanting the whole orchard.