I was IM/Rheum '92 before coming back to complete PM&R 2 years ago. Couple of things:
1. You would have a 3- rather than 4-year program
2. It's worth it to do the entire residency, because you've got to know the principle of rehabilitation. NO SHORT-CUT here.
3. You will be able to moonlight, so income is no problem
4. There are sport medicine fellowships for IM, but they are precisely that: focus is disease management in sport-age population, rather than injury treatment
5. Prepare for total switch of focus in patient approach.
6. Prepare for possible clash with the training staff, since you have been out as a practicing MD. Please keep in mind that having a combined residency w/ IM-PM&R is not the same as having functioned as IM, then do PM&R. I did have quite a bit of problem (no-one's fault)during the training, because my assessment of patients (ex: electrolytes / hyponatremia issues, or lupus-induced strokes) were at a more sophisticated level, and I don't like fiddling around and be meek. But you are there for a different specialty, not to do your own. However, it still is your responsibility to make sure all patients receive appropriate care (I was told when it comes to a lawsuit, you are liable if you have a Board Certification in a specialty that is related to the patient's problem, even though you are a resident in a different field, you are expected to RECOGNIZE problems, and ask for help--the OATH!). So that was added stress, but it worked out.
7. It's worth it, and PM&R is a great specialty--not because of lifestyle (I work long hours), but because it provides a missing link in our training.