Perhaps the reason you haven't gotten responses yet is because it's a rather broad question and it's hard to know where to begin without knowing more information.
eg, How far along are you in your psych training? What rotations did you do during your non-psychiatry months in intern year?
Anytime you're switching between specialties, the basic process is more or less the same. You have to decide if you're going to try to find an opening for a PGY-2 position (basically a vacancy left by someone leaving the program), versus go back and apply for a PGY-1 position.
There are so many family medicine positions in the US, you'll probably need to start by narrowing down some characteristics (geography? type of program?) and then start directly contacting programs you're interested in, and asking if they have an opening.
As far as getting credit for the training you've already done--that's up to the program you transfer into, and the specialty board. If you've done the standard psych intern year with 6 months of psych, 4 months of medicine, and 2 months of neuro, then I would expect that you can probably get credit for the medicine, the neuro, and 1-2 months of psych. You probably won't get credit for the whole year.
One of my friends from residency transferred from psychiatry to internal medicine. They basically gave her about 6 months of credit (some inpatient medicine, neuro, and 1-2 months of psych) but she still ended up having to go back and start all over again as a PGY-1 because she couldn't get credit for the whole year.
hippiedoc13, thank you for your insight and advice. To make my situation more clear, I matched this past march into Psych and I am having second thoughts on that. I feel like I am going to miss the medicine I spent four years learning and from what I am told, a lot of psych illnesses are addressed by family docs. I finally figured out that I like variety and the versatility FP physicians have. If anyone has insight on such a move based on such rationale feel free to add it here as well. I am sure alot of people benefit from these posts as I have.