I would think MS would be smart enough to track the old key and see that it's in use...I mean, they are charging quite a bit less for the "upgrade" version. With that, I'll place my bets on: it'll work at first, but soon caught by their "genuine" nonsense, and of course be against the EULA.
But I would try and see what happens...as arguing with MS's outsourced employees is always fun!