Sooner or later this will be necessary. I owned a house for many years that was built in 1952. I remodeled and added to it in the 1980's and the new plumbing was copper. I wanted to replace the old galvanized, as well, I had a somewhat more difficult task than you, since my house was built on a slab and the galvanized was embedded in concrete, so I had to run lines through walls and ceilings where none existed before, which I did progressively over several years. I still remember when I ran new copper from the meter to the house. The galvanized I took out was about 40 years old, and when you looked down a 10' length of it you couldn't see daylight at the other end because there was so much corrosion. If it would have been a human, it would have been dead from arterial sclerosis!

My advice is to go for it. You will save yourself a ton of money, compared to having a plumber do it. I don't know what your skills are, but this was the project where I first taught myself to sweat solder...and I still enjoy doing it. Your project sounds very doable, and you can do it over a period of time. You will probably need to cut into some walls to get at the various valves (toilet, shower, bath, etc.) that are plumbed to the galvanized, but it will be worth it. And when I sold the house, it made a difference to buyers that there was no galvanized left in the house.

Regarding your waste lines, probably nothing is necessary.

Thanks for your two cents, kudzu9! I think you're right about tackling it a little at a time, since it's nothing pressing. It will give me something to work on over this winter and the next.