I had a slightly odd variant of this last night.

2-class 763 swapped for 3-class 763. First not for sale as I asked at the lounge when I got my new seat.

At the gate employee +3 family members (they were listed on both the standby and the upgrade to Business lists), asks Gate Agent about First. Agent says they are not selling it, and the system shows no availability to sell - zero'ed out.

Passenger explains she knows how to do that - goes around the desk, taps the computer and 5 minutes later she has four F boarding passes.

She, her husband and her two teenage kids enjoyed their seats in F whilst the other passengers were squeezed in to the tiny seats on the 3-class 763.