Originally Posted by LeonS

I booked SEA - LHR - SEA for August 2021.

(1) I booked on alaska.com for ~$1,037 (it was ~$40 cheaper to book the itinerary on the Alaska site Vs aa.com; also in case I need to cancel I'd rather have the cash in my Alaska wallet)

Both flights showed plenty of C available on ExpertFlyer.

Adding the BA connecting flights to Spain was increasing the ticket cost by almost $500 for departures at times that were not convenient to me. I'll book separate flights myself*.

(2) Called AA airlines (zero wait; agent was the friendliest agent I've had on the phone on a while). She said she had already done a few of these Alaska-gifted upgrades since yesterday so she knew what we were doing. She said I was the first case where the flight was not booked on aa.com directly, so the process would be "just a little different" because of the codeshare. She said that she had to re-issue the ticket (so I am now fearing that if I need to cancel I might after all not be able to get the cash back into my Alaska wallet...)

(3) I had to pay $66.30 for "Luxury Task" for the return upgrade, originating in London. Less than I expected.

And that's it! Both flights in confirmed Business class.