I just used Finnair to change my BAEC number to my AA Platinum number on a BAEC award reservation and it seems to have worked as it always has. I logged out and logged back into ba.com, and now see the AA number on my reservation. So unless they have changed something that will overwrite it in the future, this still works fine.

Edited to add: I now realize I may have misinterpreted the question, as the OP may have had an AA award booked with BAEC, not a BA award. My answer was for a BA award, though it is not totally clear either way what he/she was intending.