I finally had someone else do it. He did in five minutes what would
have taken me an hour because I would have been too gentle with the
pipes and such. I could have done it though, and now that I watched I
know what to do next time!
It took a while because the new one is smaller than the old one so he
had to get the various parts to have it fit. I would have used flex
pipe for the gas line though just to make my life easier.
So far, two days, lots of hot water, and none on the floor. And
heating season officially began yesterday as it finally dropped below
68 in the house and the heat came on. Just in time since the furnace
gets the water from the water heater.
Someone pointed out that the reason the furnace gets the water from
the water heater is likely to prevent cold water from flowing in when
the furnace is hot and cracking the block. That makes sense,
particularly since there is an automatic water feed which usually does
kick in when the furnace is running.