Telephone AA and ask them to change the booking code. They will probably charge you the phone booking fee.
Sometimes if the ticket isn't purchased you can put it on hold, then phone AA and have them change the fare code, then you can finish the purchase online without the fee. Unfortunately I've tried that a couple of times and ended up having to talk on the phone to AA web services, since the software is too smart and keeps wanting to override the 1-point fare with the "cheapest available" 1/2-point one. Eventually I've gotten it done, but not without a bit of hassle.