With a cartridge filter, you won't be losing much water on backwash. So, CYA levels won't drop on their own.

To go from CYA=100 to CYA=50, you'll need to dump 1/2 of your pool, and refill. The only other way is to allow the pool to get 'slimed' over the winter. CYA usually bio-degrades, to nitrogen gas (no problem) or nitrates (usually not a problem). But sometimes, it biodegrades to ammonia, which is a big problem. So, that's not really recommended.

If you do drain, be SURE to do so ONLY when the ground is reasonably dry. Even concrete pools will float out of the ground when emptied, if the ground water levels is high enough.