Wonder Boy: The Dragon's Trap (mentioned around the middle) is IMHO a fantastic example on how to modernize an old game.
Not only they reused the original source code, but you can switch between the original and remastered graphics and music at any time, and even separately in any combination. It also allows you to generate a passcode so you can use your "save" with the original game, on top of being able to accept original passcodes as well.
It was a true work of love; the main coder, Omar Cornut, wrote one of the major SMS emulators and this was his favorite game. I suppose that talent like that is hard to find for every single game, though...
Edited by Newsdee, Sat Apr 13, 2019 5:38 AM.