Pipe replacement is in dad's future. There are three choices of material. Copper is, in my opinion, best. However, depending on accessibility problems, copper may be impractical for a repipe job. My second choice is PEX. Much easier to get through walls, but does require special tools and techniques to install. Last on my list is CPVC. About the same problems with accessibility as copper, but it's just cut and glue. Requires frequent support to keep from sagging. Unfortunately, houses are not built with the idea that new plumbing will someday be necessary, so often some walls have to be opened. Both PEX and CPVC need to be one size larger than the galvanized because the internal diameter of each of these is smaller. You can replace 1/2" galvanized with 1/2" copper, but PEX and CPVC would need 3/4", and 3/4" galvanized would require 1".