Is it possible to take multiple majors in a college or university?

Yes, you can usually change your major, and no one is going to force you to choose the major you put on your application. You don't have to choose your major for sure until you've finished the first two years of college! You can usually double major (for one degree in two different subjects) and some colleges let you get a dual degree (two degrees in two different subjects, can often take six years instead of four.) Adding a second major may require the completion of roughly the same number of academic credits as getting a second degree. The UCs offer dual degree admission on a case by case basis (not sure about CSU, ask an admissions counselor). UCs and Cal States both offer double majors. USC (a large private school in California) offers dual degrees and double majors.

Changing your major might change your class schedule depending on what majors. For example, a science degree to another science degree might not change your classes much, but something like a Biology major, to, say, a Theatre major definitely would. It all depends on the classes required for your major.