Replacing water heater gas control valve

Question

First let me summarize my questions, then provide detail:

Summary:

• Can a newer gas control valve be installed on an older water heater?
• As an individual who has never attempted something like this and never worked with natural gas am I completely insane for even contemplating a DIY project?
  • Read details below in regards to why I am considering replacing the valve myself.
• Also seeking general guidelines / instruction on replacing a gas control valve, especially anything specific to my model.
  • Fair number of YouTube videos that make it look easy
  • My model is an A.O. Smith GCV 50 100
• Water heater still seems to be in decent condition; a replacement valve should get me at least a few more years (and hot water until the real problem can be fixed)
  • There is no brown water indicating tank corrosion
  • There are no apparent leaks from the tank
  • The water heater also already has an expansion tank installed on it. You can barely make it out behind the flue.

Details:

My wife and I bought a house last summer that is now 10 years old. Our natural gas water heater gave up the ghost last weekend.

Let me show you what my utility closet looks like:

Note the AWESOME freon line that runs directly in front of my water heater, therefore preventing it from being easily replaced without rerouting the line. It could probably be done, but that runs a risk of damaging my HVAC.

I was quoted about $1600 to replace the water heater by two different plumbers; one says he can "manhandle" the water heater out, the other wants the line rerouted before he will try (quoted me $1475 for the job). I'm afraid that the "manhandle" route might end up "manhandling" more than just the water heater.

One plumber also quoted me $700 for replacing the gas control valve. I already replaced the thermocouple on my own and I don't think it was ever part of the problem. I should definitely buy a multi-meter...

I can purchase the gas control for about $200 and I am confident that I can make the repair, however I am nervous about working with natural gas.

Here is the gas control valve now:

So can I use a newer gas control valve or should I get an identical replacement? Should I spend the $2000 + dollars to replace the entire thing or attempt a repair on my own, have the freon line fixed and then wait for the tank to go?

I am looking for instruction, direct advice and opinions more informed than my own.

Here is the model information for the water heater as well:

Answer 1

So I did it! And it turned out not to be the problem...

Things to note:

• You may need a rubber mallet to remove the old gas control valve.
  • I had to bang on the T junction pipe to the left of the valve to get it unscrewed

• Here is the joint compound that I used

  • 

• I also used teflon tape.

• Start by wrapping the teflon tape into the threads of the compression fittings 2 or three times
• "Frost" the threads of the male ends of the fittings liberally; a little too much joint compund (pipe dope) won't hurt
  • Try not to get it inside the pipe however, so stay away from the very very end of the connector

And of course, be sure to turn off your gas when working with it, drain your water heater if it is an older style model (like mine) and be careful putting everything back together.

My problem: My problem ended up being the thermocouple. The replacement pilot light assembly I had installed just 8 weeks ago ended up having a faulty thermocouple. Obviously I should have taken more time to test the thermocouple rather than spending all the time and money on a new valve. Ah well! I'll be putting in a new anode shortly and hopefully I will get another 3-5 years out of this water heater. Enough time to take care of the freon line when the budget allows...

Also worth mentioning, I found an identical gas control valve from a vendor on Amazon. Ever since I got the new pilot light assembly in place it has been working just fine.