I have booked a multi-leg flight on AA.com that is over a variety of BA (AA codeshare) and AA flights.

On AA.com I can change my AA metal seats without issue. On BA.com the website errors whenever I attempt to enter the seat change section of Manage My Booking (I'm Silver so would expect to be able to do so?).

I have called BA and provided them with both the BA record locator and the ticket number, but they are unable to change my seats - saying this is because AA allocated them.

I then called AA who claim that I can only choose the seats on the BA flights at checkin.

This doesn't sound right, but am I wrong? Is there anyway to change the seats on the BA flights?