Welcome to FT!

On the day of the flight, if there are still unsold seats on the flights you want, I'd say your chances are pretty good - especially if you're at the airport. At the last minute, all the advance yield management calculations go out the window. They don't much care which plane you're on, and most AAgents (all else being equal) would just as soon make a good customer happy.

If you're eligible, I'd suggest checking in at a first-class position. The folks there are a bit more used to the idea that they should take good care of people.

(If it was a paid ticket, rather than an award ticket, many of us here would suggest staying with the original itinerary for the extra miles!)