The cool thing about this is that every fitting is brass NPT. The copper pipes are flared and have brass nuts on them. That makes it very easy to swap out the copper for plastic by introducing NPT-to-plastic brass fittings.
The line I replaced fed the hot water for the shower. It was 3/8" and was reduced to something smaller just before it got to the shower. The fittings I got at Lowe's reduced the 3/8" pipe to 1/4" ID plastic. You can tell by the reduced hot water flow at the shower. So, I'll probably re-do it. But for now it's all sealed up so I'm happy with it.