Actually it is not up to the individual program. The RRC has standard guidelines that regulate what would be given credit.
Besides what the RRC dictates, as stated before you must do at least two years in the same FP program. Therefore the best possible scenario would be a minimum of two years in the FP program. I don't think you would get credit for a whole year of FP based on your ER residency.
If you truly want to do family medicine, I would start a FP residency, try to get credit for what you can, but in the long run not worry about it. Most FP residencies are not too terribly demanding. It would just suck having to do residency over again. However, if FP is what you want it will be worth it in the end.