A few years ago I encountered a similar issue with a major hotel chain. I had booked 5 nights and later needed to drop one night due to scheduling and travel changes. I tried to do it online (which is how I had booked in the first place) but it told me there was no availability and I had to call their reservation centre. I did, and found out that the way their reservation computer system worked behind the scenes was that when a modification was made the system was actually booking a new reservation, then cancelling the old. YOu had the same reservation number, so the customer had no idea that is what was done. Most of the time it worked just fine; but if there was no more availability for that room type or at the hotel as a whole there would be a problem. The customer service rep was able to do some things behind the scenes to make the modification actually work -- I am not sure exactly how she managed it, but it took about 20 minutes to pull off.
It is possible Disney's computer system now works in a similar way (or maybe it always did but they were more flexible before and are less so now).
SW
It is possible Disney's computer system now works in a similar way (or maybe it always did but they were more flexible before and are less so now).
SW