You can access the booking on BA website using MMB by entering the Sabre locator, but if it is less than 24 hours do it by chosing the option for new bookings through travel agents and it will do an extended search. IME I have not been able to change the FF # to BAEC if AA number is already in the system. You have to call AA, BA cannot do it.
If you access the booking while it has your AA number you will likely not be able to select/change seats. I have had mixed results. Once the BAEC number is in you should be fine.

Once the BAEC number is in you can also use MMB and the booking will no longer appear under your AAdvantage reservations.