Originally Posted by DrBernardo

The other bit of experience that people have had in the past is that BA FF information can be extremely "sticky", and difficult to get rid of in BA bookings/systems.

Hence unless you particularly need the BA card in there for eg outbound advance seat selection, I'd be tempted to retro-claim the sector you want to credit to BA (so fly it with no FF info in the booking) and then add the AA details before the return...