If you have been doing PT billing for years I think that taking a course or training class now would be boring for you. There really are not all that many things that you would need to know. Internal medicine billing is very similar, just different CPT's. The insurance issues are basically the same meaning that you need to verify the coverage, copay amounts etc. Internal Medicine does not usually require auth or referrals like PT does. Aging and follow up issues are the same, etc. I think you will have an easy transition.