Rowling typically stuck to having it planned out, at least the direction on where the ending would be. She admits to having some continuity errors now and then (as we know, the "bad at maths" is among the most popular of those admissions).
As for plot lines one major example was changes she made to Chamber of Secrets which were moved to Half-Blood Prince:
Certain crucial pieces of information in book six were originally planned for 'Chamber of Secrets', but very early on (first draft of Chamber) I realised that this information's proper home was book six. I have said before now that 'Chamber' holds some very important clues to the ultimate end of the series. Not as many as six, obviously, but there is a link.
She also admits to "Ghost Plots" to which some details may have leaked into the published works but the idea was never fully fleshed out or dropped:
This [Ghost Plots] is a personal expression, which has nothing to do with tales of the dead.
Over the seventeen years that I planned and wrote the seven Harry Potter books (not to mention Quidditch through the Ages, Fantastic Beasts and Where to Find Them and The Tales of Beedle the Bard), I generated a mass of information about the magical world that never appeared in the books. I liked knowing these things (which was fortunate, given that I couldn’t stop my imagination spewing it all out) and often, when I needed a throwaway detail, I had it ready because of the background I had developed.
I also found myself developing storylines for secondary (or even tertiary) characters that were superfluous to requirements. More of a wrench were the plots I worked out for some much more important characters that had to be sacrificed for the bigger story. All of these I inwardly termed ‘ghost plots’, my private expression for all the untold stories that sometimes seemed quite as real to me as the ‘final cut’. I have occasionally been in conversation with a reader and made mention of part of a ghost plot; looks of consternation cross their faces as, for a split second, they ask themselves whether they have accidentally skipped twenty pages somewhere. I apologise to anyone I might have accidentally wrong-footed in this way; the problem is, literally, all in my head.
