Is the definitive answer available on this question: you have a confirmed upgrade; flight is oversold in Y, and they are moving people to the front. Presumably by status.

In that case, does the confirmed upgrade gets replaced by an opup, or do they opup someone who does not have a confirmed upgrade?

I recently experienced the following scenario: I have a confirmed upgrade. I check in online, and the BP has the mention "cert." however I have no printer handy so I only print my BP at a kiosk at the airport. New BP does not have the mention "cert." And the flight was quite full BTW.

When boarding, however, agent asks for a cert. I mention that the BP does have the mention. But agent answers that his computer says so.

Last time I had a similar scenario, at LHR, agent did not ask for a cert.

So, what are we dealing with? Bug in the BP system which does not print "cert" when it should? Wrong agent, who looked at the wrong place? What else?