Something to keep in mind is that the date that you pick your courses determines ONLY which courses you get into, not what cores, labs and tutorials you are in.
Then Mugsi/Solar assign you a schedule, and one night at midnight they will re-open with the option to try to change around your cores, labs and tutorials.
If you are trying to change around your cores and stuff, be prepared for the SOLAR shipwreck that inevitably occurs every time more than 5 people try to log on at once. Translation: Unless you are very very lucky, prepare yourself for close to an all-nighter fighting with your computer.
Hope this helps