If I was in somebody else's body, I'd have their memories and sense of identity, so I'd think I'd always been them, and wouldn't notice any change at all. For all I know it could be happening all the time.
That's assuming that the mind is a phenomenom arising only from the complexity of the brain, but I'm pretty sure of that.
OTOH if my brain had been transplanted overnight, I'd probably be quite freaked out.