I was able to make a BA Avios/Executive Club booking and remove my BA Executive Club number and add my AS MileagePlan number using Finnair's website. The Finnair site said it had an error removing my BA number and adding my Alaska number, but the Alaska booking showed that I had been assigned a seat in Premium Economy and also showed me appearing on the upgrade list for First Class, so I'm crossing my fingers, but hopefully everything will work just fine. The AS website let me add the booking to my account, but the AS website itself won't confirm that my AS number is on the reservation, and when I try to add it, it won't let me because it says that the full name provided by BA doesn't exactly match my full name on my AS MP account (one of those 'JOHN MR DOE' looking names that isn't in the correct order, which I've often seen in airline booking systems).