From a blog today:
I confirmed with Alaska Airlines that elites can now select preferred economy seats and are eligible for Main Cabin Select upgrades — for now you’ll need to call in to Alaska to be assigned these premium seats, but online selection will be live on Thursday, August 24. Meanwhile, Alaska plans to inform elites of the change next Wednesday, August 30.
https://thepointsguy.com/2017/08/ala...seat-upgrades/