Just use enough of the old water to keep the rocks submerged and fish in another. Ditch the sand and rest of the old water. Have fresh sw
waiting at your house to fill the tank with and extra for wc
's and acclimation of the fish. Put the rock back in the tank (keep the time the rock is out if water to a minimum) then add new sand. Your going to have to drip acclimate any livestock to your new water before adding them to the tank.
Edit: keep a close eye on params and have water ready for wc
's should you get a spike