Yes, drain the water and replace first. The current salt content of your pool cannot be read by the SWG and we will not know if it functions properly until the salt is in the correct range. Is your pool plaster or vinyl liner? The composition of your pool surface and the water table where your pool is located will tell us how much water you can safely drain at a time. It may take a series of partial drain and refill cycles to get your pool in the correct salt level.