Thank you all for all the good info. Much appreciated. But you guys are scaring me

But do you need a 50 gallon?
- It's for a 5 family house with 2.5 bath. Current set-up is natural gas.
The contractor charged $55 installed for a new shutoff valve. I priced the valve at just over $5 at HD.
Maybe I will get some extra stuff beforehand that may be needed for installation and return what's not used.
@Mikeeastman - Many thanks for the instructions for flushing it out. If it wasn't so old I would definitely try this first.
It doesn't appear that you have much to think about.
. You're right about this, but I'm glad I asked the question here since I got a lot of very useful info.
bringing the exit vent piping up to code for a gas unit
Keeping my fingers crossed that it will not be needed.
The additional point about having it pass an inspection if requested when you sell is also important
Excellent point. When I think about it, I actually had a friend that when selling his house, the water heater didn't pass the inspection so he had to replace it anyway.
I spent about 4 or 5 hundred more dollars than my DIY neighbors
DIY vs installers: With my bad back, I wouldn't be able to get the new or old heater into the basement, so in my case the installation $ is preferred vs physical thearapy $. Plus, I need to save the back for when it's time to move the subs
