You don't have to change cams, but to see/feel the full potential of a CDI Box's gains you should change cams. Most, if not on all ATV motors the HP output drops the last couple hundred rpm. When you change the cam along with the exhaust and intake you are increasing your HP output so you can effectively use the extra 1000rpms. If you add an extra 1000 rpm's to a stock motor, you will actually lose speed and power at the top of your rev limit. But the advanced timing which leads to better throttle response is more noticeable than your loss of top speed and power.(This is at the very top of your rev limiter, you don't notice any power loss. You will only see it on a dyno machine)