It's GA discretion when the run the list, but keep in mind that T-30m is the checkin deadline for domestic flights. If there are any potential no-shows, the GA is going to have to do some offload and reassigning at that mark anyway, and might choose to include running the upgrade list as part of those duties. Also, someone could technically SDC, purchase a new ticket, or get rebooked onto the flight up until that point. The final run of the list at the gate is manually initiated by the GA, not automatic.