If you need a specific fare class that's higher than the lowest available main cabin fare when booking on AA.com, you have 2 options. (1) just call AA reservations and make the booking with them, and tell them what fare class you need. There will be a phone booking fee, I don't know if they'll waive it. (2) Call AA web services (which is separate from reservations) and work with them. If you can put the flight on hold, then they can change the fare on the back end and then you can purchase the ticket, but you have to watch to make sure the system doesn't kick it back down to the lower fare. If a hold isn't an option, I'm not sure exactly how they'll handle it.