I've recently begun reading the Harry Potter series, but having already seen the movies I know more about the plot than any first time reader really should, and this has caused me to notice a few odd things. As far as I'm aware Rowling never planned to get seven books out of this series, and yet a number of things come up in the first book that have me either suspect that Rowling got her ideas from rereading her earlier books or that they were edited back in later. For example, Philosopher's Stone has Sirius Black, Hagrid's expulsion, Fantastic Beasts And Where To Find Them, and the dragon in Gringotts, mentioned very early on, despite many of them not being of importance to that book.
Long story short, did any of the books receive any noteworthy edits after publication? I'd check an original copy against the version that I'm reading, but I don't have access to one.