Once you decide upon a major, you can apply as an internal transfer into your major of choice.
Transferring into any major is not guaranteed and depends upon seat capacity, student academic performance, and other Department-specific transfer requirements. Since most careers in health care require advanced degrees to practice, many students opt for an undergraduate major which allows them to establish a strong baseline understanding of the health care field and explore general interests, while collecting necessary prerequisite courses to apply for graduate school in a specific discipline.
Once you declare a major, you will register with the students in your year. However - many majors place holds on courses for the first few days while their students enroll into their required classes. So, if you know what major you want to be, it's within your interests to declare as soon as possible so that you will have priority registration for your new major.