Originally Posted by
JAXBA
If the booking is being made from your PLT AAdvantage account, your AA number should be transmitted to BA automatically. Then simply access your booking on AA.com, which should link you to BA.com - or go to BA.com/mmb and retrieve your booking there to choose your seats.
Shouldn't even need talk to anyone or update any frequent flyer numbers or anything more taxing than looking at the seat map and making a decision. 
Yes, but if the award booking is made with Avios, then the FFP number will have to be changed -- either by calling BA, or by using the manage-my-booking function at finnair.com.
