Was EM program director for 21 yrs (ACGME program), currently EM core faculty (amongst other roles). Went through this process with several residents over the years (we had a few folks complete IM, Med-Peds, or Peds residencies prior to starting our program). I've attached the current policy (which comes from ABEM- the American Board of Emergency Medicine) which governs applicants getting credit for prior training.
It spells things out pretty clearly. You can't get any credit for EM rotations (unless you did an EM rotation at an accredited EM residency program where your FM was at, and there are limits). So don't count on any credit for EM rotations. Like another poster said, you'll be needed particularly for your EM months, which is where you really need to learn, and it's the most important part of your training. I was always hesitant to give credit for ICU months, I felt that was really important to get at our program as well.
It's entirely up to the PD and what you negotiate with him/her. The max credit is 12 months, but I would doubt you'd be able to get anywhere near that from an FM residency (many of which are outpatient heavy). (The policy changed from 6 mos max to the current 12 in February 2015). With our folks it varied. The Med-Peds grads got the most credit, straight IM and Peds got less. What you get credit for has to match up with something in the EM program's rotations/curriculum. For an FM grad, you could probably get credit for your OB rotation, and likely some elective time. Beyond that you have to look at the program's rotations and see what might match up. It's all negotiable with the PD up to the max # of months.
VERY IMPORTANT- the PD MUST make the request to ABEM before you start the EM program (most likely July 1 start date). Otherwise they won't take your application. Most seasoned PD's know how the process works. Funding for a 2nd residency is a separate issue, but it's not zero funding (it is less), and some programs won't look at applicants with prior training. You will need to be a strong applicant and have very good reasons for switching specialties.
Best of luck with the process. Feel free to PM me with questions.