As someone who switched specialties, I can tell you that you don't have to take more medschool classes. You will have to complete a residency in the new field. This may or may not involve doing another intern year depending on what you are switching from and what you are switching to. Having switched from surgery to IM, I had to repeat an intern year, although some programs offered me credit for six months (the maximum allowed by the ABIM). I chose not to do this, however, as I felt the full intern year in IM gave me a better experience. You would have to check with the specialty boards involved to find out how much, if any, of an intern year you would have to repeat. For example, if you were switching from IM to gas, you wouldn't have to do another intern year as gas requires some sort of preliminary year. On the other hand, if you were switching from OB to IM, you would probably have to repeat the intern year. Hope this helps.
Take care