What Salty said. It used to be possible, decades ago, but now any flight training incurs a regular active-duty obligation.
I also know a guy who started in the guard, and then transferred to the same airframe in the Navy Reserve.
Generally its seems once you get mil wings, it's at least theoretically possible to switch between reserve branches (depending on their needs). Be aware that army helo pilots who did helo-only training (no T-34/T-6) would might need to attend UPT to get FW rated before joining another service.