Hi Tom,
I have tried to top work my Honeycrisp espalier tree, which did not fruit for me, when it was 7 years old. It was top worked to replace the rung above 3 ft.
Just like any other top work grafting process, the grafts took but the espalier tree does not look as nice as before the top working. Maybe it’s the way I grafted. I was desperate to get something that will fruit in my climate to take and didn’t really care at that time how the tree would look.
I also grafted different varieties onto individual lateral branches on the remaining rung. Because of the grafts which are already 1-2" above the rung, these are not expected to fruit as close to the rungs as they are supposed to if not grafted.
I am no expert but would just like to share my experiences.