I can tell you that if the section is not filled by fare paying passengers, it will be filled by upgrades to give the impression that the section is desired. So upgrades are handed out quickly and quietly at the gate or even before. As far as upgrades go, each airline is slightly different
Now I know why Continental went bust, flew a DC-10 trans Pacific with them and was the only first class fare paying pax on board, accompanied by about a dozen FA's, on time off, on their way to Sydney.