Need to kill donors? To the best of my knowledge, it's theoretically possible to basically mature a human body including a potentially-functional brain, while keeping that brain in a vegetative state the entire time. Of course, that's still a potential human - the vegetativeness needs to be reversible for this to be useful - so the ethics are still highly questionable.

That's how I pictured it, yes. At this point I wouldn't concern myself with the ethics of it, because, if our technology advances this much, then simply the fact that humanity can perform such a feat is an extremely positive thing, and probably the end of death as we know it. What worries me more is that this wouldn't result in a functional mature individual. For instance: in order to develop the muscular system, the body's skeletal muscles would have to experience some sort of stress, i.e. be used. If you grow the organism in a jar from birth to consciousness transfer (as is probably most ethical), it wouldn't have moved at all its entire life up to that point, and would therefore have extremely weak musculature. What to do in the meantime, electrically stimulate the muscles? Maybe, but it probably wouldn't have results comparable to natural usage. Besides, there are probably many other body subsystems that would suffer similarly without much you could do about it. See Gunnar Zarncke's comment below.

On the other hand, if you use a clone body that's the same age as the transferee, it wouldn't shock me if the personality didn't actually change significantly; it should basically feel like going under for surgery and then coming out again with nothing changed.

Yes, but I imagine most uses to be related to rejuvenation. It would mean that the genetic info required for cloning would have to be gathered basically at birth (and the cloning process begun shortly thereafter), and there would still be a 9-month age difference. There's little point in growing a backup clone for an organism so soon after birth. An age difference of 20 years between person and clone seems more reasonable.