This will sometimes work....we've had it happen and have the onboard agent tell us it was impossible. What you CAN do is to make a second reservation for the cruise with all the benefits of onboard booking and name the same TA. Then, when you get home, contact the TA and ask them to "reconcile" the reservations. They can recapture your original cabin number, etc. and cancel the less financially desirable reservation. It is easier if the onboard agent can do it for you (they have to contact the agency for permission to do it), but our more recent experiences have been that they will not.
IF the price of the cruise has increased significantly, it may not be in your best interest to rebook. Bring a copy of your current reservations (I know my mind gets hazy while cruising).