"More ethical" I still agree that we feel this more ethical to harm other species, just like it could be less ethical to harm family than a pure stranger.
Human is an animal. It is an Ape from the beginning.
And the first thing that the monkey that actually got its head transplanted on another monkey body was to bite a nearby surgeon, before to fall unconscious and die. So I guess it was painful.
It was not a head exchange, and the monkey that loss its body was not saved.
Would it be ethical to you if 100 Houyhnhnms[1] got cured from paralysis if hundred human lost their head?
Of course, I'm not aware of any Houyhnhnm existing, so this can not be applied.
I do not imagine any way to face these situations.