I had an issue last year when just trying to modify a trip due to a snow storm that blew in unexpectedly the night before we were meant to travel. I wanted to just have us arriving a day later and add an extra day on to the end knowing I would probably have to book at another resort or in another room category. We already had 7 nights booked I thought, we can keep 6 at WL and then do the 7th at another resort. Easy. Nope. They were going to have to completely cancel the trip and rebook the rooms, they could only do 4 nights at WL and the other 3 would have to be somewhere else (much more expensive btw, some of the only rooms left were CL and it was going to cost us $1500 more than what we had already paid). It is just not as simple as it would seem. We ended up just keeping our reservation as it was and booking an additional night on its own somewhere else so we could use all of our park tickets, much cheaper that way. As others have said, if you must book a reservation just to have a room book one that you are willing to pay for without a discount, or wait for discounts to be released if you just have to have a certain room. It also allows for that room category you want to show more availability so there could be better discounts if not as many rooms are booked, you booking for two weeks could make disney go "well we have people in those rooms, we don't really need to discount them as much", it's why I book value and then wait for discounts in deluxe, I have never had a problem getting a room I want that way.