That’s one of the reasons why some report they were able to upgrade way after 10:30, while the common consensus is that you must be first in line in the terminal to upgrade.
At 10:30, available upgrades are rooms for which cancellations are known to
DCL in the last three days before sailing. If someone misses the plane and never shows up to board, that will become known to DCL after some if not most guests are aboard.
Another possibility, although rare, would be that a room is reserved for a DCL executive or some other VIP. For example, we sailed with the president - now retired - of DCL a few years ago. If they cancel at the last minute, I’m pretty sure that upgrading someone to that stateroom would not go through the supervisor desk at the terminal.
I’m pretty sure, for Concierge staterooms, that your best bet would be to mention it to the hosts when you meet with them after boarding, maybe not by asking if an upgrade is available but that you wanted a 1BR and couldn’t get one. The will always try to offer those kinds of things, like a released cabana, to Concierge guests first.