The first few Christmas Parties I went to (1997-2001 or so), the MK would close at 7pm but the Party officially ran 8pm to 1am. I'm not sure at what time you could enter the park with your Party ticket only, but as it approached 7pm they would usher arriving Party guests into Tomorrowland where attractions were open for Guests with wristbands only, and you could pose for your free Party photo. (That shows you just how long ago that was) At 7pm they would do an accelerated park sweep to clear the park of day guests.
When the rest of the park was clear, they'd open the park to Party guests, which was typically before 8pm, sometimes opening Fantasyland first but only accessible by looping around from Tomorrowland. They'd keep the bridges from the Hub closed off. Then by 8pm it was Party guests only.
Perhaps they're going to try to go back to that format, just an hour earlier. It will be interesting to see whether the enter-at-4pm-with-Party-ticket still holds.
-Rob