From your post I can not tell if you are a practicing FP or a resident.
My advice: Does your residency program have an OB program? If so you would be able to go back and train perhaps for only 3 years. If you go to a different program you would need 3.5 years.
The second option is to do an OB fellowship one year which would train you on doing only OB and most simple gyn procedures D&C, Hysteroscopy, Cesarean Deliveries, Tubal sterilizations.
If you are currently a FP resident you may lose some time but the transition is much easier.
If you are an attending the advantage of retraining in a career is that you can make extra money by moonlighting in FP.
Good luck
D