Most of y'all, I assume, have far more experience with this than I do, so I defer to you. That said: it appears that more gets read into this statement than is actually indicated by the statement, the key phrase being "for selection by any passenger". As far as I'm aware, passengers can never select complimentary upgrades. They have to be processed by the gate agent. By my reading, this statement doesn't preclude the gate agent from upgrading based on the queue, regardless of boarding status - much less, require to gate agent not to process upgrades for already boarded passengers.