Yes, turn off the power and water supply. Open a hot water faucet then open the valve to drain the tank. The valve then just screws out of the tank. Of course, in the real world those valves may not want to come out, but hopefully it will. I'm sure you know that 20+ years is far beyond normal life expectancy for water heats, but as long as it is still working for you, it's sure worth the price of a new valve.