Disney has a rule that the rate that is in effect when you check-in is the rate for you entire stay. This sometimes works to your advantage, if you can check-in on the last day of value season and extend your stay into peak season, you end up with value for the entire stay. In your case, the seasons are working against you. If you make one reservation, you will pay peak for the entire stay. You can make two reservations though, one for the time during peak, and another for the time during value season. If you make everyone aware of the situation, especially at check-in, you most likely won't have to change rooms, just re-check-in for the new ressie.