I've had no problems in the past, as recently as three weeks ago, in doing the opposite on an AA reward flight, changing the FF number on it from the AAdvantage number whose miles I used to book it to my BAEC number so I can eke out my last remaining BAEC Bronze / OWR benefits before they go away at the end of the month. I've done this three times now over the last year and never had any call centre agents refuse or even argue about it, although they do seem a bit confused that I want to do so. So I'd say that if they're happy to change the FF number on an AA award booking from AA *to* BA, I can't see that they wouldn't do the reverse.