If it is an AA flight, I believe that American have stopped free seat selection even for elites when paying by AVIOS. Also, a couple of months ago the Finnair website stopped allowing you to change the FF number. It lets you go through the process, and retains your new FF# but does not override the BA number in the booking made on BA.COM. Boo!
If it is an AA flight and you call AA, you may get lucky and get an agent to override the FF#. I had the same issue on an AVIOS AA flight and the agent had to spend more than 5 minutes changing the FF# as the system would not let him do it the normal way.
Seems like the noose is always tightening on these "free" tickets.