I've been told by several reservation cast members that I could do that and I believe that I was able to do it on one occasion. The thing is is that it seems the minute those new rates come out, they are already sold out for the new low rate.
This is the case for us on our upcoming trip. We decided to go up a day earlier and we are already going to be staying at two deluxe resorts for two weeks, so we figured just to stay at an all star resort (cheaper) the extra night. I made ressies, then the next week the $49 rates came out. Of course I called and they were already sold out for that rate. (Wait a minute those rates just came out today, I said to myself. How could that be.) Well, that's what they said. I was told to try and call back 5 days prior to arrival incase anyone canceled for that night. You have upto five days before arrival to cancel and still get your deposit back.. So on Tuesday morning that's what I'm going to do. I have also been trying this past week.
Sorry for such a long reply.
I hope this helps.
Diane