Please don't worry, there is plenty of time!!! You can change your mind as far into 2nd semester i think, but depends on your tutor. I think you'd be better waiting till you get here to worry about it though, there will loads of other things for you to worry about when you start in Sept, Cassm will feel quite insignificant in comparison!
Not to scare you or anything
The letter maybe trying to match you up with certain tutors and mentors so that people's interests are catered for...dunno just a guess!!