Originally Posted by
number_6
That is an involuntary reroute, and you get the benefits of that; much higher priority than missing the flight by failing to check-in on time. At a minimum it is the next AA flight out in the same class of service, often it is better than that.
It depends on whether they are on one ticket or separate tickets. (Why separate tickets? Because it was cheaper, or because they were booked at different times for whatever reason, or because ...) On one ticket, they have to protect you. On separate tickets, how you get there for the second leg is your problem - not theirs.
It has been posted that linking the PNRs gives you some protection even with separate tickets. I can't vouch for that. Perhaps someone else with specific knowledge can post here.
As for the original question: the "flat tire rule" generally applies if you show up at the airport within two hours after your flight was scheduled to depart. However, that AFAIK just gets you standby priority for AA's next flight. No top priority, no rerouting on another carrier with an earlier departure unless they really like you.