I agree customers should get what was published at the time they paid for the service product.

I had a similar issue with ANA. I specifically bought ANA coach seats due to 8 abreast seating on the 787 and the corresponding seat width.
Then they introduced premium economy and changed economy to 9 abreast. I was not notified and happened to notice the equipment change before I checked in. I complained and asked for Premium Economy, the next class of product that meets or exceeds what I paid for. Of course I was denied and told I got the "class of service" I paid for...

Magically overnight I was upgraded to Premium Economy (maybe my Star Gold status helped to op up me?).

So, I have a personal issue with the advertised specs of the product being changed after you purchase the product.

I think Alaska needs to proactively notify every customer who has purchased Preferred Plus on an upcoming flight, notify them of the change in the product, and offer them a refund.