CYA won't change unless you drain and refill a large amount of water. Once you've tested CYA, you can use that number the whole SLAM, as long as you're using liquid chlorine to SLAM! If you use a solid shock product of some sort (please don't do this), or still have chlorine pucks anywhere in the system (floater, chlorinator, etc - please don't do this either!) you may need to retest CYA to get the newer, higher, chlorine level you'll need to keep SLAMing.