I would think younger coaches would be more likely to switch from either side, but once a coach has begun to develop his program and they are having success, usually that coach is gonna stick with them or at least with that side (girls or boys), and usually they can't see themselves coaching the other, even though they probably could.
Also, just my own opinion, but I would think it'd be more likely that a boys coach/assistant would move to coach a girls varsity team. Didn't Western see something like that? Not sure of the exact story and maybe I'm wrong but didn't one of the boys' assistants take over the program or at least move to help as an assistant with the girls and that's when they started to turn into a power in their own right?