Myself and one other ExP were upgraded by an FA after departure when this exact situation occurred on a MIA-LAX flight a few months back. The GA was apparently too busy to take 5 seconds and process the upgrades so the FA took matters in to her own hands and took care of us. I think the problem in this situation might be that there were a large number of open seats and I don't know how an FA would be able to discern who gets moved up front. I will say that the FA on the MIA-LAX flight had the passenger manifest in her hands and told me and a person sitting in the row behind me that we "were clearly the next 2 in line for an upgrade" when she allowed us to move. She also apologized for the incredibly rude GA that couldn't be bothered to upgrade anyone - I actually asked the GA why the flight was leaving with 2 empty seats in FC when he came on to close the door and he told me that "it happens all the time, too bad" while the FA was standing right there listening - I think that's why she moved us! This situation is one area where AA needs to do something... I have been very happy since moving to AA from DL (long-time DL DM MM) but one of the things I always gave DL points for was that their GAs would never leave a FC seat empty if there was an upgradeable passenger in coach and it seems to be somewhat common at AA.