Last year, I initially added my BA FF number but have Emerald Status on Alaska so wanted to change it which you can not yet do on the BA site. I used the Qatar Airlines site to change it. You can look up your OW reservation there but inputting your BA confirmation code. Once the reservation has been found, you can change your FF number to Alaska. Save your changes then go back to the BA site and it should now show your AS FF # which should allow you to make free seat selections on all BA flights. Some seats are blocked specifically for BA elites until 24-48 hours before departure but all other seats should be selectable if they have not been already.