You can come in to one of the FiOS LPC stores at any time to exchange equipment. We always have new hd boxes in stock, the latest revision at that. The only thing we sometimes lack the latest model of are the DVRs. Call your local store, they'll tell you if they have what you want or not.
The only thing you can't just exchange for free is the Gen 3 router, revision I. That'll be a 79.99 charge and there's no way to waive it.