My experience was that the app doesn't let you change the seats if the booking wasn't made via AA. I used chase points to book STL-MIA-SXM. AA removed STL-MIA, and rerouted us to STL-CLT-SXM (no notification, just happened to be looking on the app for US-EU mileage tickets and noticed the STL-CLT). When I opened the app, we'd been booked on the late STL-CLT flight (short overnight), and put in the nonexistent 3A and 3B on the 319 to CLT-SXM. When I tried to change it, it said I had to request changes through the agent who booked the original ticket. It took an hour on the phone with the Chase agent having to be the go-between us and AA, but I finally got rebooked to the only open F seats, 1A and 1B, and as a bonus, was able to get an earlier flight to CLT without paying anything.

Honestly, their "shortest duration" default rebooking algorithm stinks. Especially with the unrealistic MCT they use now. Almost guaranteed misconnect and IDG.