I have an existing booking for later this month in WTP which I've MFU'd one leg. Thus far I've always collected Avios on BA flights.

Since booking, BA and AA have come up with bonus promotions, albeit with BA's existing bookings do not qualify (no such restriction for AA).

How do I go about changing my earning preference for this booking (only) from BA to AA? Does the fact that I've MFU'd a leg impact my ability to make this change?

Ta muchly.