The CYA dilution test can get kind of tricky when diluting/testing over 200, so I would err on the side of caution and retest once you do a healthy water exchange. Of course never completely drain a pool, but if you chose to exchange say 50% at a time, you can retest to see how the CYA has changed. I would ideally try to get the CYA under 80 if you are able based on city ordnances and such. Adjusting the CYA to a more manageable level will allow you to be certain of the FC/CYA balance as noted on the [FC/CYA][/FC/CYA] so that you can close properly and avoid algae over the winter.