In the past we've had room-only reservations (these did
not include a dining plan package though, that might make a difference

) that were already paid in full when an AP rate became available and when I called about the new rate they just went in and modified the ressie - the difference was credited to our credit card pretty quickly.
I have also changed resorts previously on a paid in full ressies where they went in and modified to the new resort and refunded $$$ to my card (or I had to add $$ if the new one was more) - in the past I have been known to change my mind, ahem, lets just say, often... just ask my poor DH. "I thought we were staying at the CR" as we drive up to the AKL
I just kept watching (and watching!) for deals and called just as soon as they became available, so I could be sure the availability in my category had a good shot of still being there....