The switch is covered by warranty. Either your vendor or JetBeam will replace it. I'm willing to bet that JetBeam would just mail you a new switch without requiring the return of the light, as long as you can confirm that it turns on when you connect the negative terminal of the battery to the body.
Also, the tail cap is the same on both models (Jet-II Pro and Jet-II IBS), so I can't see why the switch would not be interchangeable.