It may not look like it, but the chances are there are a lot of movements in the flights - passenger numbers change by a degree which isn't that visible on public systems. Moreover in terms of shuffling between cabins, any staff travel on that service can be manipulated, including, in many cases, manipulated off the service altogether.

If you want a POUG then it's best to check in as soon as possible, however it's usually AUP that your chances are strongest. It's difficult to influence this since ultimately the computers know about you, and everyone else, in greater detail than you know of yourself. So it's best to go with the flow and to accept whatever happens.