In every state that I know of, a medical license permits you to work as a physician in every field from psychiatry to foot and ankle surgery. You get your license after completing medical school, step III of the boards and an internship. In New York, the medical licenses say "Medicine and Surgery", no matter what you specialized in. In fact, you don't even have to specialize as long as you did you internship.
The difficulty in doing what you have suggested comes in other areas. You will have great difficulty getting hospital privileges to do anything except what you are trained in.
That being said, family practice does not require much hospital work so a shift to it should not be too much trouble. Of course the laws may change it 20 years too.
One final thing, the call schedule for OB attendings may not be so bad if you are in a medium to large sized group. If you are one of ten OBs, you would have call 1 week night every two weeks and 1 weekend every two and a half months, that's not bad at all. My old man is an OB and before he retired he was in solo practice, so he was always on call. He worked 4 days a week in the office, did rounds each morning, surgery 2-3 mornings a week and delivered babys when ever they came. He was at virtually every little league game and band concert for me and my sibs.
Ed