I have been out in the world for the past 5 years at NYE and my understanding is that the parks fill to capacity in stages ie firstly some guests may be turned away if the car park is full, then they are closed to all non hotel property guests and then to all guests if filled to capacity. If you exit a park you are not guaranteed re-admission.
Normally the MK fills up first, then Epcot and then The Studios. Last year Epcot did close around 4.30ish but it may have re-opened later on when young families may have left.
We exited MGM to go to 'Cirque' at Downtown Disney, and had no trouble re-entering at 10ish, but that is the park which usually fills last. We have never done PI, because the age limit is 21years on NYE and we still have teenagers.
This year I am not sure what to do. It can get VERY COLD as Hilary said, and I don't know if I can face any park from early a.m. till 1.00am. We may have a lazy day and just try to get in somewhere for the last few hours. Simon will of course be at Epcot, but as he will be working hard there seems little point in hanging around there just to see him LOL Sorry Simon!!!!!
Just be prepared to wrap up if need be. Last year and other years we have had to scrape the ice off the car before we could leave. Carolyn