I can't imagine Jim using Time Travel to rewrite history entirely. I mean, if he goes back in time to, say, GP, saves Susan from becoming a vampire, etc... It basically means that the entire plot of several of the books just doesn't happen any more, why bother reading them, just skip them and let the recap in the book fill you in on whatever you missed.
Now, it MAY change how we view some of the events that have happened, another perspective on motivations, whatnot and so forth. Find out who was behind certain mysteries, and the like. But for changing events as written in any fundamental way? I doubt it.