I did do this once - we did it in Oct on the Magic for the cruise we just completed on the Dream. What they did was book us a new reservation on board with the on board booking benefits, then when we got back, our
travel agent moved the room we wanted to the on board booking and cancelled the original booking. It was a savings of about $50, but then there was the on board credit as well. Piece of cake. (HOWEVER, we ended up getting Florida resident rates for it, so that negated everything.)
Reshop and let them know you are doing it when you sit down with them. What you need is the total price of the cruise (the fare, not including the taxes) to compare to see if you are saving any $. What I recommend you doing, is if you do go ahead and book the reshopped cruise, name a travel agent at the time of booking. That way, you can get the rebooking on board credit, and the travel agent's as well
For 7-night cruises, we get $275 from DCL and $200 from our travel agent. That pays for our tips, Palo, and puts a dent it anything else (excursions, drinks of the day, etc.).
Good luck and let me know if I can help you any more