I don't think this is a subject you can generalize about. These days the way some think about classic courses (depending upon what's meant by the word "classic") is that if a particular architect did the course then it's a classic.
A good example would probably be if you have a Donald Ross course--any Donald Ross course--it should be considered a classic--or should've always been considered a classic and never altered. Not true, in my opinion.
My course--Gulph Mills G.C. is a pretty good course--and one that Ross promised the founders in 1916;
"....one of the best inland courses in this country and that it will undoubtedly be a much superior course to any around Philadelphia."
Uh, Donald, it seems to me you may have been overlooking both Merion and Pine Valley right here in the Delaware Valley!
Nevertheless, over time--about 15-20 years the membership concluded that approximately 3-4 holes of Ross's original course weren't all that good and they were altered, very likely for the better.
Merion wasn't actually completed by its architects until maybe 1930-34 and PVGC wasn't really completed until about 1922 but after that neither one has been architecturally altered--certainly not butchered, modernized, scarred, disfigured, ruined, etc, as Pat listed in the first post on this thread.
Why not? Probably because they always were considered classics--sort of like courses such as Cypress, Shinnecock, Seminole, NGLA etc that were also always considered classics and basically left alone because of that.
Seems to me what some people are doing when they call some courses classic is really calling the architect of that course classic. To me, anyway, it's not the same thing.