negative mold hydroforming. with a thick balloon and a 3d printed mold.
it works for plastic, it works for steal, it should also work on wet leather too right?
so this would i think only really be a good idea if you already plan to do a total rebuild, but for a crazy project this seems kinda viable.
if one could 3d print a mold that fits the outside of the leather and also closes off where sole would be, you could use a thick rubber balloon filled with water so you can very slowly reshape the boot atleast a little bit into the shape you want.
besides being extremely niche from what i know this at the very least should work right? it likely will effect the reused leather negatively, but thats alright depending on what you plan on doing. it might also actually help a bit in restoring the original shape for the rest of the boot