Yes, just did it 2 days ago.
Had no problems
It so happened to be a short trip too.
It was a simple error on my part
We booked a trip to go to the AP preview of Guardians ride
I meant to book Sunday through Tuesday but accidentally choose Sunday through Wednesday
I caught it yesterday so I went in and selected Modify reservation, changed check out day from Wednesday to Tuesday and completed the check out process. Had no problems with removing the day. Promo was still available for same resort and room type. While you go through the check out process again, there was no change in rate or anything. Now, if the check in date changes and that date has a new rate then yeah, you'll pay a difference (ie, you go from arrival on Thursday to arrival on Saturday, weekend vs weekday) but in my case, removing a night didn't do anything but reduce the total due at arrival, the deposit remained the same so I checked out with a zero balance due. Hopefully, that makes sense. The check out process is needed to finalize the change.
It never hurts to check. Nothing is changed until you actually go through the final step of the check out process.
If you go in and select Modify and the system shows the resort with no rooms, or with no rooms in your same category then yeah, it's sold out. You can always call and ask for them to make an exception. They may or may not do it