Joel,
most of the restorations of classic courses in continental Europe have been incremental, with one or two holes of a few bunkers done per year. Examples are Kennemer (Colt), Eindhoven (Colt), de Pan (Colt), Spa (Simpson),
Exceptions have been Royal Hague (Colt/Alison), where all 18 greens, greensurrounds and all bunkers were redone in 2 years time. Other courses with wholesale changes were made in Hilversum (Colt), Falkenstein (Colt), Royal Antwerp (Simpson), Frankfurter (Colt) and Club Zur Vahr von Limburger).
Also in the UK more and more clubs are looking to restore many of the key elements of their classic courses (strategy, fairway width, tree clearance, restoring bunker style etc). Tandridge (Colt) where I am working is a good example of such a large scale project, restoring the course back to its roots in 4 years time. (we are lucky to have a lot of good historical photo's of the course just after opening).
Jay,
leaving the courses alone helps, in that many of the classic courses still have almost their original greens. Only a few classic courses have in recent times changed/lost some/all their original greens. Hilversum (Colt), Frankfurter (Colt), Wentworth West (Colt) and Club zur Vahr(v Limburger) are sad examples of that. I know how frustrating that can be; we have such a situation at de Pan where three original greens are changed two decades ago, which in two cases has not worked out, but we have no way of going back to the originals because nobody bothered to accurately survey the original greens before they got changed....
However even doing nothing for many years has still left many of these courses with far too many trees, too narrow fairways, tired looking bunkers and lost detailing around the greens. Even a great place like Morfontaine suffered from some of these symptoms when I visited a few years ago.