Room rates are only guaranteed so long as no changes are made to your reservation. As per WDW terms and conditions:
CHANGES TO RESERVATIONS: Changes to a reservation, including but not limited to travel dates, length of stay or hotel accommodations, are subject to availability when the change is made and the guest is responsible for paying any increase in price resulting from the change.
ROOM RATES: Confirmed room rates are guaranteed as long as your reservation is not changed (except for changes in tax rates or other governmental charges).