Shut down water to home and gas to water heater. Drain down portable water system and 40 gallon gas water heater. Once drained detach all lines and remove. Place new 50 gallon gas water heater in pan. Install new three-quarter inch progress ball valve and furnishing install 2.5 gallon thermal expansion tank. Secure to wall. Make all necessary connections and pressurize system slowly. Bleed out all air from new water heater before lighting.