Originally Posted by
swag
...
As far as downside, on AA, once you check in online, you are no longer able to change seats until you get to the airport. If a better seat opens after you check-in, you're stuck where you are. And if you check in for coach and then your upgrade clears, you can't select your F seat either, until the airport.
Similarly, I checked in at T-24 on a AA award ticket then got the idea I'd like to extend my trip. Called the AAdvantage Desk and although there was availability, they could not make the change because I had already checked in. Had to wait until my outbound was complete, with the chance the availability would not be the same at that time. Fortunately, it was. Since then, I still check in early, but not *that* early.