One more idea, albeit probably not worth the time and effort, would be a topology agnostic transfer mode. Basically something that can repeat the same UV manipulations done with one model for a changed variant of it, assuming the islands are at least roughly in the same spots.
It would be interesting to see how notably the new textures change from the remap. The examples shown get a bit more blurry, but they also seem to have been scaled down as a result of the combination. A pure rearrangement would help to identify changes (if any) better. A no-save demo is a great idea. A lossless / pixel…