In my experience, the seat map goes to gate control at the baggage check-in cutoff. i.e., in Atlanta, you won't be able to change a seat via OLCI, airside kiosk, phone app, etc. within the 45 minute window. Since upgrades are often cleared 35-40 minutes prior to the flight, one may need to speak to the GA, who will likely be busy with other requests at that time. This is less of a problem at departure airports where the cutoff is 30 minutes.

Keep in mind that once upgrades are cleared, standbys are cleared. It seems that more often than not, standbys go into preferred seats.