I've always tossed out the old water. My feelings are that old water is just that, old dirty water. You will stir up so many contaminants that it would be pointless to put dirty water back in, even if you suction it out as clean as possible before disturbing anything.

Believe this or not but I've had really good luck helping people move with this othre trick I once saw. Put about 3 gallons of sand to the side in a bucket. Then take the rest of the sand and wash it out with tap water. Rinse until clean. This helps prevent fouling the tank with all that buried detritus. Spread the clean sand first and cover it with a thin layer of of the saved substrate to quickly reseed it. Your live rock should have plenty of microfauna to help establish the tank.

This may or may not be sound advice but I've used it to move 4 tanks over 100g that were fully established and each time there was barely a hiccup once set up.