You can apply for an upgrade from your current booking done at American Airlines Flights whenever you want but before a few hours of flight departure. You just have to speak to our team and talk about an upgrade. You can opt for an upgrade provided it is available once we check with our team. We usually do not charge you anything for upgrading your seat. However, you will have to pay the difference between the prices of the tickets. However, an upgrade is subject to availability but you can request one if you want and our team will make sure you get one once you are confirmed. You can check out these and other details at American Airlines Official Site.