QUICK TIP: Did you know you have until September 12th to add/drop and change around your course schedule? (
http://registrar.mcmaster.ca /course-adddrop-dates/)
Once classes start everything isn't set in stone. In fact, the add/drop period runs right up until September 12th. That's quite a long time, and also gives you a chance to go to your classes, see what they are like, talk with an academic advisor (there are advisors for each faculty), and get things sorted out. So long as you don't drop a 'required' course, since you need those for your degree! If any 'required' courses are full, or conflicting, then your faculty will open up a seat for you, or make it work. Don't worry. As for 'electives', you get to choose which you want, and can add/drop them up until the 12th. Good luck!