Note: I do this regularly, so I am looking for a faster option than calling the airline on the phone - I'd like something I can do online.
I have booked a reservation with American Airlines on aa.com. It is ticketed and paid for, and contains an American Airlines flight and a BA flight. I am American Airlines Platinum (Sapphire) and the number is against the reservation. I would like to view the reservation on the BA website to change the seating, which I can't do on aa.com.
I know from past experience that such reservations tend to have two reservation codes, one for the AA portion (Sabre?) and one for the BA portion (Amadeus?). But I don't know how to find the BA code, or to display the reservation on the BA website. Entering the AA code simply gives an error that the reservation cannot be found.
How can I find the BA reservation code for this reservation, or change the seat?