re: Gas water heater, how hard to replace?

Any competent DIY with a helper can change one out. Good advice in this thread and youtube.

The reason plumbing companies charge so much is the city makes them pull a permit and do an inspection and invariably there are issues like catwalks or other stuff that come up in the inspection process.

Why the hell you need the city involved in a water heater replacement is beyond me.

I would make sure you have good access in the attic and do some plywood improvements on the catwalks if you don't. This could save putting a foot through a ceiling by accident.

Also it isn't that hard or expensive to properly solder copper joints. I'm not a fan of sharkbite fittings but many seem to like them.

Post up some pics and a writeup here when you're done and let us know how it went!