Finnair does not work anymore. It stopped last year sometime. I just call AA now, give them the flight number(s) and have them switch the frequent flyer number to my AA one. When you do that the trip will no longer show up on the BA website but will show on the AA web site under their version of the PNR.You can then select seats for free. Ask the agent what the American PNR is, and keep a note of both the AA PNR and and the BA PNR. They use different systems hence the need for different PNRs. Anyway, I have never had any issues after changing over to the AA PNR and FF number. I am also One World Sapphire via AA, but only blue with BA.
Update: Thinking some more about this, I have always done it when booking an AA flight on the BA website using Avios, so not sure if it will work if you are talking about a BA operated flight, but maybe you could just call BA and ask them to change it in the phone.
Last edited by Furby; Apr 5, 2019 at 10:44 pm