The "Smokey the Bear" principle--i.e. "leave the campground the way you found it"--tends to apply more to books based on TV shows that are still in production. In general, you tend to have more freedom once the actual shows are no longer going concerns, simply because you no longer have to worry about stepping on the shows' toes.
Case in point, when I wrote my first 4400 novel (while the show was still in production), I had to be careful to make it a standalone "freak of the week" story that didn't affect any of the show's ongoing storylines. But when I wrote the second one, after the show was cancelled, I had much a much freer hand to kill off characters and such.