In my "highly valued opinion" :wink: , Phantom's Revenge is the first to spring to mind. Steel Phantom was heralded as the brink of roller coaster engineering when it opened 1991. Which it was - though it was the fastest roller coaster in the world and tallest roller coaster to feature an inversion, it would thrash riders giving a very rough, uncomfortable ride. It could have faced the same fate of other large Arrow loopers (Shockwave, Great American Scream Machine), but gratefully Kennywood looked in 2000 to convert a massive multilooper into a hyper coaster.
Morgan was hired to do the conversion, and certainly did a heck of a job. All of the track except the lifthill was replaced, and the layout after the second drop was heavily altered to do airtime hills circling Thunderbolt instead of the original loop and batwing.
The Steel Phantom was ok, but its reincarnation as Phantom's Revenge has truly been a turn for the better.