Short answer? Almost anything. The American higher education is structured in such a way that students have a lot more mobility overall. There are a few reasons why this is the case:
You don?t just take classes in your major
The US higher education system comes from the tradition of the liberal arts; rather than studying one subject exclusively, most American college students spend two years in a four-year bachelor program fulfilling ?general education requirements,? taking classes in math, science, social sciences, languages, art and more. This is supposed to make you a more well-rounded person and develop various skills, such as speaking, writing, analytical skills and critical thinking. (This is also why you can change your major in an American college without 'starting over.') American universities believe that this diverse educational background prepares students to study nearly any field in greater depth at the graduate level.
You have to re-apply to graduate school
You can?t just enroll at a university when you?re 19 and keep studying for six or so years ?till you get a master?s degree in the USA. Instead, you apply for and enroll in a bachelor?s degree program (which takes four years to complete) and graduate. Then, you select a new university and degree program (usually not the same place where you did your bachelor?s) go through another round of applications and are admitted to a master?s degree program, which generally takes one or two years to complete.
Professional school only exists at the graduate level
You can?t get a bachelor?s degree in law or medicine. Some people who want to study medicine major in biology and are known as ?pre-med,? but lots of other people study music or history or chemistry or whatever and then apply to medical school. You can get a bachelor?s in business, but you don't need to in order to get an MBA, and some might advise you to study something else at the undergraduate (bachelor) level to make you more well-rounded.
Caveat: You might have to take a few courses.
Some master?s (and professional) degree programs do require you to take certain courses before you apply-- the are called prerequisites. Medical schools, for example, generally require biology, anatomy, physiology, calculus, and two years of college chemistry. Check the university's website for the master's program you want to apply for. If in doubt, email an admissions representative and let them know that you?re interested in applying, and that you would like to know if there are any courses that you?re recommended or required to complete before admission. If you already have your bachelor's and still need to take a few prerequisites, you can usually complete these classes cheaply at a community college.
If you do decide to pursue a new field at the master?s level, be sure to emphasize how your previous field gave you a unique perspective or skills that will be useful in your new field. Give clear examples of experiences that led you to your new field. It?s also good to show that you have a real reason for picking your new field, and that you have career plans.
So frame it like this:
?While studying history, I began interviewing immigrant families about their experiences and became absolutely fascinated by struggles they faced. I wrote my bachelor thesis about migration history, but I wanted to do something more concrete, so I decided to pursue a master?s in Migration Studies so that I can work for a nongovernmental organization that serves refugees.?
Not like this:
?I started in history, but it?s boring, so I decided to study migration because I heard that you can make a lot of money with an MA in Migration Studies.?
The bottom line? If you're not in love with your current field, consider getting your master's in a new one!