There is a hierarchy for upgrades. Generally they keep the seats open for sale until close to flight time, usually 24 hours. Then the computer crunches the applications for points upgrades. They get allocated to the highest tiers of FF first, then if unfilled to the next tier etc. If there are any seats left over they then get offered in the bid/pay upgrade option. Any left over are then sometimes offered as a pay for upgrade at the airport. The algorithm for AC is somewhat quirky. Once I put in for the bid for upgrade option. I really wanted it, so bid the maximum allowable amount. I didn't get it, even though I knew from the seating plan there were empty seats. Strangely I was then called to the desk just before boarding and was offered the upgrade for 50% of what I had bid previously. Sometimes I think they keep a few seats open if they think there might be a problem with connecting flights because of weather etc, and they might have to find seats for people who missed earlier flights.