We have heard that when you are lowering CYA, it may be possible for CYA that has become stuck to the walls and equipment to go back into solution as you drain and fill. At least, this is the proposed explanation for the effect we see of an X% water drain and fill not resulting in X% drop in CYA.
However, I have noticed that the CYA measuring tube seems to be on a log scale, so that a 50% reduction, for example, will take you from CYA 60 to CYA 35. Since there are no markers for CYA 100+, it is hard to tell what a 50% reduction might be from say 200 CYA.