I don't think the case study scenario changes. I was there on the first interview date and have heard of my case study being reused on two other occassions. Basically, it's the one about a woman getting sick on a cruise ship. The case study consists of 3 parts, each part building off the last one. It has kind of a surprise ending.
I've also heard that there are people who know ahead of time what the ending is and it becomes obvious who those people are. MY friend interviewed there several days after my interview. He came back and told me that within the first few minutes of the first part of the case study, someone was already going...in a very bad act..."wait guys...I think the symptoms might be pointing to ____. I mean, it could be that she is _____ because I know that ___."
anyway, like I said...surprise ending, but this person was guessing the surprise within the first few minutes skipping over all the obvious crap. Anyway, it doesn't matter if you get it right, all that they want to see is that you can work in a group, follow the steps of PBL and not be an dingus to the other group members.