I don't understand, how does the body not instantly reject the foreign substances. And how do you change height/bones/muscle-structure. Stem-cells aren't enough. Is there nanotechnology? If this is a "magic" form, then yeah, a fair bit of balancing is in dire need. Making it a science-fiction however could open up alot of paths (Though I'd rather you didn't, I'm working on a MASSIVE techno-mythological saga, and nanotech plays a large part in it, as does body augmenting and things)
Also, here's another idea. Perhaps you can make it like a sort of dual-time reality. Sort of like Schrodinger's cat. Except, both here and there. So like, you tune into the other person's body, and they are you and they are themselves. You retain what you look like, but it appears as if the other person is you. And as such, their clothing is both there and here, so people will feel the texture of it and everything. And they will feel people touching them as well, hear the things you hear etc etc so it works both ways, except the person who you're impersonating won't look like you, and won't know where you are etc. I reckon this works alot better, as people would be able to tell if they are being Bodyjacked, and then they can be intuitive and listen to speech, chatter etc etc to figure out where you are and what you are doing etc etc.
damnation...now I want to use this idea...