but ABC wanted more
Some of the old cast is still around, Dr. Cox and Turk should appear in most episodes, J.D. should appear in about half, or something like that, I've also heard that Kelso and Elliot will make appearances, no word on Carla..
The Janitor will only be in the first episode if i remember correctly.
They have moved on to a Medical School, Turk, Dr. Cox and J.D. are professors teaching there, and there are three new cast members, and one who was promoted to cast.