As a backup give the FA a copy of the upgrade list so a plane never leaves with empty seats. I had this happen to me. I was sitting in the baulkhead, I saw the empty FC seats and I was waiting for the GA to come on and do the upgrades. That was a few years back. ON CO now on flight status you can see how many FC seats are open and the upgrade priority list. Only one issue, they do sell FC seats at the last minunte giving
people the opportunity to get a confrmed FC seat at a cost.