Can I change my major once at college if I did NOT put “Undecided” on my application?
Yes. A huge percent of students, even those who list an intended major, switch at some point in their college career.
You can and many students do. The only time it could be an issue if if you want to switch into a major that would require you to be accepted into a different division of the school (ex. engineering, business, nursing etc.). If you want to make that type of switch I suggest you contact admissions and see if they can move your application to the school/division you now want.
Just so you understand, the fact that you put, for example, ‘math’ as your potential major on your college application doesn’t mean that you have ‘declared’ math as a major. You just told admissions what you think you might want to study. To actually declare a major at a school, you need to file with the school registrar usually (or the dept.) And most people choose an academic advisor in the dept at the same time, and it’s usually someone they had at least one class with whom they like and respect. That’s why most people don’t ‘declare’ a major immediately.
This question depends on the school.
At some schools, or some divisions within schools, students are admitted declared into majors. At other schools, or other divisions within schools, students are admitted undeclared, so that they take frosh/soph level prerequisites and then declare their majors later.
At some schools, declaring or changing major is merely a formality if you are in good academic standing and have taken the needed course work to be able to graduate in a reasonable amount of time in the new major. At other schools, some or many majors are oversubscribed or impacted, so they may require a higher GPA or a competitive admission process to declare or change to. Note that the latter may also apply to changing to a different division (arts and sciences, business, engineering, etc.).