Data point involving booking a cheap flight a day after changing my chosen airline:

7/28: logged into my Personal Platinum account and chatted online with an agent to request changing my airline for the airline fee credit to Southwest since I hadn't used any of the credit this year; while still in the chat, I received a confirmation email with my chosen airline clearly stated as now being Southwest, and I also verified the change shortly after ending the chat by navigating to the Rewards & Benefits section while still logged into my account and seeing that my chosen airline had been updated to Southwest
7/29: ~28 hours after updating my chosen airline, I purchased two one-way "Wanna Getaway" tickets (on the same PNR) for $58.60 each
8/1: meter moved
8/2: 2x $58.60 credit posted