Originally Posted by Jaimito Cartero
It depends on whether the gate agent is up to speed or not. During very busy times, they may forget to check the waitlist, and flights may go out with empty FC seats.
I usually just say, "I just wanted to check if my FC upgrade had cleared", just before they call my row for boarding. As long as you're pleasant, and the GA isn't too harried, it often works out.
Once you board, there is only a slight chance they'll pull you out. (I'd guess 10%). Sometimes the scanner will "beep" differently when you're going through to let the agent know that your ticket was upgraded.
It also could depend on the FA. I was on a recent flight, in 1B. The GA came on board, just before closing the door, and told the FA "the pax in 20C can take 6C if they want" (757-300). The GA then closed the door. The FA opened the cockpit door, and told the captain we were ready to go, without even offering the FC seat to the pax in 20C. I guess he didn't want to take the time, effort, etc..