Hi, Christian! During the height of the Christmas season, it is not uncommon for a park to close because it has reached capacity. Once this happens, they cannot allow anyone else to enter the theme park.
If you are hoping to do some park hopping, you will need to be aware in advance that it may not be possible to hop to the park of your choice if that park has already filled to capacity. The days most often affected by something like this include Christmas Eve, Christmas Day, and New Year's Eve, but it can happen other days during this very busy week.
Cast Members are notified if a park is closing due to capacity, so you can often ask at the park you are in before you leave. Buses will be aware of park closings, and they usually make an announcement so that people are not disappointed.
Just a side note, even if you are in a theme park early in the day but leave for any reason, if the park fills to capacity, you will not be able to return to the park you were already visiting.
I hope this helps with your planning. Even though it's busy, this is one of the most magical times of the year to visit Walt Disney World, and I'll be right there with you, soaking up the holiday spirit!