I may be wrong but I think it depends on whether or not your origional booking was a room only or a package. If it was a package, you should be able to change the rates if you are still w/in your cancellation period (and they have a discount rate available for your hotel/dates). I'm not sure what the cancellation policies are for packages, but I do know they are A LOT stricter than for room only.
If you booked a room only, I'm not sure that you can do this. However, if they have package rates available for your hotel/dates then you should just be able to book that and then cancel your room only since these reservations can be cancelled w/no penalty up to 6 days from your arrival date.
Also, you probably already know this but you should compare costs between booking separately and booking a package. Mary Waring (mousesavers.com guru) says in The Unofficial Guide to WDW that it is almost always cheaper to book separately.
Hope this helps.
