In an interview with Time magazine, Rowling says children who contact her "are always most worried I'm going to kill Ron. They've watched so many movies where the hero's best friend gets killed."
The books have the traditional storyline of a battle between good and evil, the young hero Harry using his powers as a wizard to battle Voldemort, killer of his parents.
"I am writing about someone, Voldemort, who is evil," says Rowling. "Rather than make him a pantomime villain, the only way to show how evil it is to take a life is to kill someone the reader cares about."
Harry Potter books have sold two million copies in Britain and more than five million in the US. A film is also is being made.
There are other changes in future books. Harry, a bespectacled innocent schoolboy, will grow up, developing a "foolishly smitten and moony attitude", and will have crushes on girls.