Our CYA is really high (100), actually, and the pool calculator suggests replacing 60% of the water to correct it. I'm not sure that needs to be done immediately (the pool guy had said some time this year). Part of the problem is he's been using the chlorine tablets, which add CYA. We're going to finish what is in there, and then switch to bleach, so it at least shouldn't get too much higher.
I have a TFP post planned to ask a few questions, this will be one of them.