After you get the AMADEUS reference code, go back and use that code to call up the flight info on BA.com, and if you are AA Plat or ExPlat, it should allow you to change the seat assignment. Note that even months after changing the seat assignment, the AA system still hadn't updated the seat info, but when I went back to BA.com to look up the specific flights with the AMADEUS reference code, it said I was still in the seats I had changed too.

An equipment change on my first flight took me out of the exit row due to different numbering on the upperdeck of the 747-400, but no complaint from me as the change was from Club World config to New Club World config--I like!

Steve