Galvanized, Brass and Copper

I have a 70-year old home and need to renovate an old master bath due to a shower pan leak. The room has been gutted, and it looks to this uneducated eye as if almost everything is galvanized steel, except for two hot and cold lines heading up to the attic bathroom, which look yellow like brass rather than dark gray. My contractor is recommending replacing with copper the galvanized down to the basement, but not all the way across the basement because the pipes down there are behaind a maze of HVAC ducting that was installed over the pipes that run across the basement ceiling.

Questions: Although I assume it's advisable to replace the galvanized (we have great water pressure, so there's no sign of the pipes narrowing, but it has been 70 years), should I worry about replacing it only vertically to the basement and not horizontally? Also, if the pipes going up to the attic from the galvanized in the master bath are brass (the attic bath was likely built 40-70 years ago) and not galvanized, should I bother to replace them? I don't want to have to break through the master bath wall should there be a problem with those lines going to the attice, but I also would like to leave well enough alone and not crack a brass pipe or create any other problem that didn't exist. Do I have a risk in copper connecting to brass and galvanized?

Thanks very much for any wisdom.

Copper and brass get along together well, no problems there. Galvanized, though, is not great.

I'd leave the brass, but replace all of the galvanized I could get to.

I would replace it with Wirsbo Pex A