In general it is easier to switch out of medical/law/engineering/business departments (sometimes called schools or colleges) than it is to switch into them, but that depends on the policies for the particular university. I know a small number of people that have switched majors or colleges. Depending on the university you go to, you may also have the option to have multiple majors, or a major and one or more minors. So it's neither a lot or not at all.
Do you already have an idea where you may apply to? If so, go check on the school's website to see their policies.
On a related note, you should be honest with yourself and list out the reasons why you don't think you will stay with it. What are the alternatives you are considering? Why did you consider it in the first place? Are there ways you can answer your doubts now?
Hope that helps.