I'm in the camp that says they will probably update the world slowly over the next two expansions rather than doing it all at once. The reason I say that is because there are two ways they can present a world revamp: there is a major world altering event that demands it (like the Cataclysm was), or they just decide to update zones so we have updated zones to play in.
If they go with the world altering event then I would say 10.0 is most likely. But if they are just going to update them so we have updated zones then I don't see any reason for them to do it all at once.