How easy is it to switch your major once accepted to a school but before you are enrolled or while enrolled but before the first day on campus?
Depends upon the school and major.
I think it can be difficult to switch to highly popular majors at some schools. You can check the websites of the schools you applied to for their rules. I don’t think it matters at less competitive schools. Some don’t require students to enroll in a major until the end of their sophomore year.
The majority of schools don’t admit by major, so switching majors once you get to the school is trivial. Switching before you get on campus may be difficult, but only in the sense that you haven’t officially declared a major yet anyway so whatever you put on the application doesn’t matter that much.
Some universities admit you by major, so switching into competitive/impacted majors may be hard but switching to non-competitive majors may be easy. And some universities admit you by school, so switching majors within your school is easy, but switching majors between schools is hard.