As much as I think you should be able to do what you want with your assigned seat, I also think the purser did a good thing here. It really does make sense for the person with the highest status (who was potentially denied an upgrade) to get the First Suite (especially if it's a flight like SYD-SFO/LAX). This is espcially true if the plane hasn't left the gate yet, and the purser knows who might still be on the upgrade list. Which then raises the question of whether UA should take an upgrade!h