How/when did you find out about it? Also did you get any other notification (usually an e-mail) from them?

If you went to the airport and got the word there then if they don't have a very serious proof (eg an email) then it's an IDB and they're now on the hook for your expenses (reasonable) including lodging and food, etc... Also there's a rather large payment due to you as well.

Did you escalate to a Supervisor and then a Manager to see what they could do? A lot of agents just do the absolute minimum and/or don't have the authority to reinstate things after a change is made, especially if there's no seats in your cabin. You'll need someone with greater authority to make anything happen.

If there was an AA FUBAR then you should get in touch with them and work something out. CS is very slow to respond, Chat and Twitter agents are usually a lot better and will almost always get the right level of people involved.

They messed up one of my reservations a while ago during a schedule change and it took a chat with a Manager to get things fixed after about a 45 minute call and being passed around from Agent to Supervisor to a Manager before they finally got it right.

If there's a connection or two involved it might just be that the system messed up with something (usually for me MCT) during the schedule change process. I've had a bunch of them where I get rebooked in the middle of the whole process when all of the changes haven't been completed in the system and the rebooking system jumps in way to early and makes an absolute mess of things and required human intervention.