The usual way to do this is to put the BAEC number on the booking, at the time of booking, and leave it. BA is an option on AS booking windows. If your status doesn't show on the boarding pass then ask AS at check-in to add your number, redo the boarding pass and you then need to keep the boarding pass for any retro claim. If you put another card on it, well I know the process, but I don't know if AS will do it. You ask AS to remove the credit from your account, wait a bit, then ask BA to credit it. It then can be sticky since even after AS have removed the credit, the ticketing details may indicate you have been credited and thus refuse to process the booking. But with persistance on the telephone you should get there eventually.