I was just #1 on a J0 flight last week. Spoke with the gate agent just before the list ran. She confirmed that once you board your name is removed and normally you won’t be sought in the cabin. She wrote down the order of the upgrade list before they ran it and she assured me she’d have someone come move me up if someone didn’t board.

I think this is still very much agent specific rather than a wholesale SOP change for AC. Sometimes you get a good one but (mostly) you just get the bare minimum.