Results from severely dehorning a peach tree are variable, depending on the age of the tree, the vigor of the variety, and the time of year it’s done.
Sometimes, if you don’t leave enough foliage at the end of the scaffold, the scaffold will just die. This is especially true if pruning in the fall, winter or very early spring.
If it were me, the safest thing to do is prune each scaffold down enough to let lots of sunlight in the lower portion of the tree, so see if you can encourage some growth there. If you don’t care much about the tree, then I’d say roll the dice and do a drastic dehorn. Just make sure you leave some foliage on the ends of the scaffolds.
What I generally tell my customers, when they ask this very question is that raising peach trees is like raising children. If a parent waits till a child is in his/her teens before any good training is applied, the child’s character is already formed, and any correctional training applied at that point is generally too late.
It can be done with peach trees, but generally easier to just plant a new one. They come into production pretty fast anyway.