Here is a data point: I had paid 110 PlusPoints for Skip the Waitlist on SFO-HKG. (I believe StW books into the XN bucket.) I had to call in to apply PlusPoints to HKG-BKK. The agent said it was difficult to do, but after putting me on hold for awhile, she reported that she was able to request an upgrade for 20 PlusPoints.

A week before the trip, the upgrade hadn't cleared, even though PZ=9. I called the 1K Desk. The agent said she couldn't clear my 20-PP upgrade because I was "third on the list". (My theory is that the previous agent had put the upgrade request against the XN fare bucket?) I asked her to release the upgrade. I was able to rebook in PZ for 30 PlusPoints. Although the increased PlusPoints cost was annoying, I was able to have a friend sponsor the upgrade with his expiring PlusPoints rather than my 2027-expiring PlusPoints.

Can anyone corroborate my theory about the fare bucket that was used for the 20 PlusPoints? Thanks!