Keep checking. We have found that AP availability fluctuates all the way up to the time we leave. There have been a few instances where we found an even better AP rate than the one we had currently booked.
As PP mentions, you can only upgrade to AP on-site, but you can still book the AP rate and just bring it back to show evidence of it after you upgrade. In fact, in more than half my stays, I have never even been asked to show evidence of the AP at all!