I can't speak specifically to California, but I can tell you that I found nursing school to be much easier than my classmates, having just finished paramedic school and having had previous EMS experience. I'd be surprised if you can challenge the paramedic boards as a nurse, unless you already have your ACLS certification, because that's a BIG part of EMS and I have yet to find a nursing school that includes ACLS as part of the curriculum (doesn't mean that they aren't out there, though). Also, from what I understand, medic to RN programs are more common than RN to medic programs, most notably Excelsior's medic to RN program. Whichever route you settle on, I wish you the best of luck!
Mike in Michigan