So weird.
Alaska Air wouldn't let me check in online because...they'd also stuck us on the next day's flight without telling us. They hadn't pulled the original flight off the PNR. There were no notes about the change.
The very nice reservations agent pulled us off the next-day flight, so we were able to check in and get boarding passes for our original flight, which is oversold.
I wonder if this was A) Alaska protecting us onto the next flight and flubbing it or B) a prelude to bumping us.