I think if we looked at all courses, and not just famous ones, as I just did, the number of improvements would be higher in general. That is because a great course probably has its holes moved for non golf reasons since it had few bad holes to start with. Most courses probably came with a few clunkers, and adding some new ones can only help if it removes those.
And I think most courses have changed routings more than we give credit for. Over time, things change. Longer tee shots, safety, circulatoin problems come to light. And in the non golf realm, roads, power lines, housing, flood damage all affect many, many courses.
Medinah as we knew it was a complete reroute a few years after opening, for example. 16 at ANGC is considered a much better hole and was a reroute and redesign. No one alive now remembers the old SFGC.