When I crashed my CF bike early last year and toasted it I got a crash-replacement frame and moved my parts over. The frame was a step up from my previous, and the seat-tube size was different. However, as the rear derailleur (a Dura-Ace) is braze-on, used with a clamp, the shop just replaced the clamp with a spiffy CF one for a fraction of the price of another Dura-Ace derailleur. So I prefer to go that way. I think it beats getting the derailleur in a big clamp size and using shims, or not being able to move the part to a new frame.

Functionally I don't see a difference so it's a no-loss situation.