What happens if I completely change my mind on what I want to major in?

No worries. It happens to a lot of students while in school. But realize that depending upon how far along you are in your courses, you may have to take  additional credits for your new major. For example, some of the biology or chemistry classes you took for pre-med may not count as electives if you change your major to business or pre-law. It's a good idea to check with your guidance counselor after you switch to make sure you enroll in the right classes to fulfill the graduation requirements .

I agree with Paul's answer. It's not the end of the world and it happens more often than you know. It's okay if you lose credits, it's okay to change your mind. A lot of what you did study will transfer over in some capacity, and it is truly fine if it doesn't. You will be upset, but you will still succeed at whatever else you've set out to do. Do not pigeon hole yourself into something you don't want to do because you've invested too much time or energy. It is perfectly fine to change your mind! Do what is right for you and what will make you happy in the long run. If you work hard and are passionate about it, the change of majors will be a blip in the radar down the road. Best of luck!