This sounds as though it was a quite an organized process and that it worked well. The only potential criticism is that passengers were upgraded after boarding. But, most people would prefer that to waiting at the gate until definitively upgraded or not.

Presuming a sold out flight, at T-60, all passengers must be checked in or risk being off-loaded. Thus, there may be some availability for SB created an hour out. At T-15, all confirmed passengers must be boarded, so additional availability is created then. On flights which are not sold out and there are no disrupted passengers to deal with, there may be additional space which could become available more than an hour out.

Thus, there are at least three buckets (check-in, gate, free seats) and the downside risk of clearing too many early is that confirmed passengers do show up and SB's, having been told that they have a seat are then told they don't. Best to wait and.

Thus, by leaving it with the GA's to work from what they can see on their monitors, what appears to be disorganized actually permits the GA's to confirm SB's as the situation progresses.