There's a long(ish) thread in the General Discussion forum speculating about both what the new project is and about how it will affect this game:
http://forum.square-enix.com/ffxiv/t...-on-a-new-game

Basically, at this point, nobody knows, and everything's pure speculation, but the consensus is that there's no way they'd sideline a game that's bringing in as much money as this one. It's more likely that they're bringing in someone else to cover some or all of the work Yoshi-P has been doing with XIV so that he can put some of his attention on the next big game they're working on. It's the start of a new fiscal year, and they're shuffling people around.