To answer your first question: The original poster was anesthesiologist currently doing his fellowship and was inquiring about switching residency to NS. He stated he would be in his late 30s after he completes NS residency. I think the main problem the OP would face was funding. There *may* be a few programs who would take someone who has already gone through residency. May be a resident could chime in on that.
For your second question:Switching from FM--->NS is *possible* but it depends on several factors. Age, board scores, funding, LORs etc.