This assumes that the GA officially upgrades you in the system before you board. When there's a risk of delay, I've seen GAs tell someone their seat or even say to take any open FC seat.

On my last gate upgrade, the GA called names and distributed new boarding passes, but after I boarded the new seat wasn't reflected as my seat assignment in the app or the app's seat map. I don't understand how that could have happened, but I'm glad that someone else din't come along and claim the seat.