You don't change the box. All FiOS DVRs are CAPABLE of the Multi-room feature. It is merely NOT ACTIVATED unless you have ordered the service. Tech Support merely activates the Multi-Room feature on your existing box. So you should not lose any recordings.
I just did the reverse, and the CS reps kept insisting that they need to send me a NON-Multi-Room box and I would need to return my Multi-Room box, but after some persistence, I finally got transferred to a guy who knew what he was talking about and simply de-activated the MR features.