Enrolling and SAO question

So today is the first day to enroll in your courses. I have a pretty good idea with the courses I want to take, and when they will take place in the day. My teacher from HS told me and my class to enroll right when they let you because the classes can fill up fast. I am also planning on going for SAO next week. Am I allowed to enroll now and if I need to make any changes, drop out?