Galvanized was good in its day. That day has passed. Do yourself a favor and replace all that you can get to behind that new shower so when the time comes and it leaks or clogs up with rust in the rest of the house, you don't have to tear out your nice shower to fix it, too. Follow those lines back until you find a convenient place to make the transition, and replace all of the supplies to the shower wall. Not sure about the best transition, but you really don't want it behind the tiled wall in a new shower - do it further back where you don't have to tear it out to fix.