Will you have to start a new game here? Not that I mind playing it again (I think I'll even end up doing it anyway, just to remember what happens in the game and because I enjoyed it the first time I played), but I still have my old save there.
Not necessarily. Depending on where you last saved the game, the saves should work. You can also use the memory reconstructor (a built-in save editor) to skip the beginning and start with the new content.
