I think the problem with waiting on the gate upgrades to clear is that the agents are generally working waitlists, request etc.. Your timing would have to be split second to grab that seat. It would probably be better to nicely ask the gate agent if you could get one of your 'perferred seats'

Alternatively stand at the gate area from an hour on and watch the "cleared list" . As soon as it starts showing first class seats try to grab a new seat?