[rwest]

Copper all the way for me and I also use the “L” because I’m odd like that!

I would cut out a section of the old with fittings and see if it has been worn down by water swirling and if not, leave it alone, otherwise replace lines that will be hidden after the work is done and leave the stuff you can get too easily later.

If you happen to have a recirc line, I would replace that as it has water moving in it all the time.