If you are planning extension work it may make sense to replace an existing installation in its entirety, especially if its approaching 40 odd years old. Copper piping will become coated in lime or sludge deposits over time and it can become "porous" with age and fittings can suffer from dezincification also. With a new installation you have an opportunity to avail of the latest advancements in plumbing and heating technology, such as thermostatic controls, zoning etc. As rightly pointed out you don't want to be ripping up new work to replace an old fitting or pipe that's just started to weep.