Data point from last month. In FRA waiting for FRA-YVR, J was booked full, I was #4 on the list, but late getting to the gate (not really, bus gate, boarding started early). Concierge came along the line looking for any SE on the upgrade list. Had the conversation that once boarded, opportunity for upgrade was lost, but that there were several J passengers connecting on short connections who might not make it. So I waited, moved to #1, got regular updates on whether J was fully boarded , until the final booked J passengers came just as they were filling the last bus out to the gate. Didn't get the upgrade, but did get very clear direction to always wait until J was fully boarded.