Originally Posted by merlin90

I am an infrequent BA flyer but in the past have been able to adjust my details post-booking via the Finnair website. I've just booked MRS-LHR next month in Y using Avios. I have OW Sapphire via my AA Platinum status. I know I won't be earning any miles from this flight but I would like to select a seat in advance without paying. (For additional benefits—primarily lounge access, if that exists at MRS, and baggage allowance, I assume I can have that changed, or at least recognised, at the airport.) I've tried using Finnair and Royal Jordanian sites to change the FF number associated with my booking but to no avail. Is this now completely impossible? Is it because it's an Avios booking, which somehow creates an unbreakable bond with my BA account? Any other ways around this?