My SEA example was with US.

The last time I flew PHL-SEA (not-very-full A319 ontime arrival/departure), the gate agent went to the back to upgrade someone who had already boarded (not free, he had to surrender 3 coupons). As said, I guess it all depends on how much time they have before scheduled pushback and the size of the plane. In my US volunteer-after-boarding example, they just called for volunteers and didn't go look for anyone who had volunteered at the gate.