They will have park hoppers, so expect to see more and more arriving during the day.
They get a private party starting at 11pm, so expect Disneyland Park to close 9 or 10pm, but California Adventure to stay open to 10 or 11. Regular guest can leave but the Grads will have to do something.
Also they no longer get Fireworks instead they get a private showing on Fantasmic. so maybe there will be fireworks before thier party starts for everyone. Maybe no Fantasmic for regular guests. More demand for World of Color.
If you are driving to the parks. They park the buses at the Toy Story Lot so it may be closed to the public.
Keep and eye on the reporting about the event in early May to see what you are in for.