Sometimes high schools push students to make career choices early is because of college application. Colleges would like to see that student's activities have a focus and the student is passionate about something. It is hard to show your passion when you are undecided.
Students do often change their mind once they get to the college. Admission officer know that and I don't think that they care that much. They wouldn't say that we admitted you as English major so you can't study Math, for example. However, there are exceptions, it is quite difficult to transfer from college of letters and science at UC Berkeley to college of engineering which is more competitive. The other direction is easy.
