I think most of the old pipe would have to come out because they would snake the Pex through the old holes. They would take it out in pieces, so that would reduce the amount of damage to walls. A lot would depend on how the old pipe was routed. For example, my plumbing is all in the basement except where it come up to the fixtures, so when I replaced the old galvanized with copper, it was really pretty simple. On the other hand, a multi-story house with pipes in walls might require some wall opening. No way for us to really tell without looking. Replacing whole house plumbing with Pex is not a DIY job, so find a professional company that not only has experience with Pex but also with using Pex in existing homes.