I was very surprised at the last minute availability when we were at disney last week. I had to change our dinner reservation on the holiday weekend, I think it was Sunday 10/10 because the kids wanted to stay in the pool longer. I checked online to see what would be available around 6:30-7pm, and LeCellier came up for 6:30pm.
During the rest of our 9 days, I was able to make a few last minute changes with good availability (even O'hana came up once).