While talking about a number of subjects with the Plumbers who replaced our water heater on Friday, I mentioned that I was aware of the old heater's slow draining bottom valve from doing a D-I-Y replacement of its gas valve 18 months ago.
(Yes, I considered doing a full heater replacement then but the heater was otherwise working fine, a new gas valve was only $100, the valve was in stock at a local warehouse which is open 7 days/week, and the valve failed on a Sunday.)
The Plumbers told me they do these valve replacements "Live", with the tank still full of water! They turn off the inlet water to the tank, use the T&V valve to remove all internal pressure from the tank, and make sure NO ONE in the home even touches a faucet while the valve is being replaced.
With the tank de-pressurized and all valves closed, the internal vacuum will prevent water discharge during the few seconds needed to extract the old valve and insert a fully-prepped and doped new valve.
I used the same technique on my car 2 months ago to replace a faulty engine temperature sensor on a cold January evening. I never thought of doing this 18 months ago when replacing the gas valve on our old water heater.
I am sure all the Pros know this technique but this is a D-I-Y forum and many readers probably do not.