@karb Of all the options, I think biological is almost impossible to get right.
I find it easy to suspend my disbelieve because "it's magic, it just works™" and "our technology is already so unimaginably complex, I guess it could exist".
But biology exists in our daily lives and that makes it harder to come up with something that doesn't contradict our view of how it works. Shape-shifting IRL is limited to very "simple" things such as caterpillar -> butterfly, and that takes weeks or months, for a body that's only a tiny fraction in mass compared to that of a human.