If you connect copper plumbing pipes directly to galvanized plumbing pipes, or even let a copper pipe touch a galvanized heating duct, you will get an electrolytic reaction between these two metals. The reaction is accelerated if the pipe is hot, so this is worse with hot water lines than cold ones. This reaction will simply put holes in the copper and the joint or the contact point will leak.
So if you need to mix old and new pipes, make sure that the connection between the two is non-conductive to electricity. This is called a dielectrical coupling. The best of these couplings is a fiberglas threaded sleeve inside a galvanized cast iron fitting. The two pipes screw in but there is no electrical path between them.