Here is a data point.
I have XN ticket on Continental metal. I have a OnePass Mastercard. I checked in at -24 hours. Two minutes later I called the Elite desk. I asked them where I was on the upgrade list. I told them that the upgrade list online showed nobody. There were 4 open first class seats. I was told that I was #2 on the upgrade list. I hung up. Within 15 minutes, all the available first class seats had been taken by upgrades (mine not being one).
So, I guess some people chose to pay for an upgrade when they checked in? I didn't get an offer to pay for one.
Any other guesses?