On a recent flight I watched 3 separate self-upgraders (economy regular seat to emergency exit row) to be told to pay up, or move back to their original seats (they all moved back). I had paid for emergency exit row, so was obviously fine where I was, but since there were 5 exit row seats, and we were positioned XX OOX (with X being occupied) I asked if I could move over to the open aisle to allow all three of us more elbow room. The FA was fine with me doing that.

With a lot of airlines, not sure about AA, once checking opens at T-24, many of those paid for reservation seats often become free to choose (emergency exit, bulkheads tend to stay pay to use).