1) Cash rooms and points rooms are different inventories.
2) You are working with different legal entities, Disney cash reservations would need to be paid for the room. What happens to the originally reserved points room?
Think of it like two houses next to each other. You paid to rent House A, but really want to stay in House B, but it is owned by someone else. The owner of House B is going to want full price to let you stay there, not just the difference between what you paid the owner of House A, and the normal price of House B.