Well... what do you mean by preserved? Like untouched by another architect? Or if it has been what changes are allowed? What if it changed over time, then all put back (like oakmont with the tree removal). Are we talking "shot values" preserved or physically the same (yardage, contours, trees and tees)?
Almost all of the old stuff has changed. If it was built in the US before Tilly, then he probably took out a bunker here or there, working for the pga. If it was built during the golden age, it was probably changed by RTJ.
You could say Pete Dyes courses are probably the most influential courses because not only are there so many, and so many other great architects that have worked under him, but if there is work to be done, it is normally pete making the changes (at least to my knowledge).