room rates are VERY unlikely to change.
However, there’s no guarantee you will be able to add the
DDP. They problem is that they can’t actually add the DDP to a RO reservation. Room plus DDP is a package (even if no tickets).
RO reservations are sold by Disney Destinations
Packages are sold by Walt Disney Travel Company
(Even through they have the same call center/representatives making the booking.
So, you would have to make a new package reservation, and then cancel the RO reservation. You may or may not be able to use the deposit from the RO towards the package. They might require a new deposit, and then issue a refund for the deposit for the RO reservations. If there is no availability for a new package reservation (meaning a room still available), you will not be able to get the DDP. While it may be possible for a supervisor to put in an override,that can not be guaranteed if the system shows no availability.
Also, by adding the DDP, thus getting a package, you would have the package terms of payments and cancelation.
In summary:
RO:
5 day before check in to cancel without penalty
FInal payment due on day of check in
Deposit is 1 night stay
Package:
Final payment due 30 days in advance
After final payment date,there is a cancelation fee.
Deposit is $200