mr.mark
Do you have a propane torch?
After doing all the water shut off, power shut off, pressure relief stuff...simply heat the joint and knock the pipe off the valve.
If no torch, just cut the pipe off with a hacksaw at the valve.
Listen, the overflow pipe is just there to divert the water to the drain pan at the bottom or to a drain.
If you run the water heater with no drain pipe, SHOULD the water squirt out (highly unlikely) it will squirt straight out the valve. Is there anything in that line of squirt that would be damaged? Discolored? Shorted out? If so, just place a deflecting piece of flat metal or plastic in front of the valve discharge.
Don't get excited...this discharge pipe is 999999% of the time superfluous.
If you DO have the torch...stick the old pipe back on the new valve, pressing inward and heat the joint. It will slip on and stay in place...leaks are inconsequential...don't even bother with new solder and flux and all. A good grip is all that is needed, there is no pressure there...it is a relief valve not a pressure valve.
Hell, you can DUCT TAPE the relief valve to the cut discharge pipe. It will hold the second or so the valve is open which is a 999999% improbable event anyway.
Problem with free advice is few people follow it. If I charged you $45 for this post you would run out and promptly fix the valve.
I know, I ran an appliance repair business for 11 years. I would give someone a free service call because they were poor, had done me a favor, or were regular customers. They promptly called someone else for a second opinion and PAID for that second opinion.
Human nature is really twisted towards self-destruction.
ola