Had the exact same situation for IAH-DFW a couple weeks ago. In that situation they DID op-up 3 or 4 pax to first after the upgrade list cleared and put those of us standing by in E -- would have cleared into the front but for a 45 minute flight wasn't too fussed (though of course would have preferred clearing directly up front prior to the opups). They didn't do that because they had to assign seats first and wait until t-15 for noshows (of which there were 4), which of course makes sense so hard to argue with.