I did this not long ago myself.
We check in next Wednesday. About 2 weeks ago decided to move to Universal a day early so I called Disney to remove a day from our stay at WL
It did not require a new reservation (my reservation number did not change) and there were no rooms showing available on
MDE for our dates so there is a way for them to do it
It did take a while, the CM at reservations said she had to contact someone else to do it, I forget now the name of the department she gave me
Will they do that every time? No idea. But they can. So it is worth a phone call. Nothing lost but your time