In the first four books, the process of Apparating from one place to another is called "Apparition". In OotP, however, and consistently throughout that book, it is called "ApparAtion". I know, because I was grumbling in my virtual beard about it the whole time I was revising D&L to be OotP-compliant -- I had to change about a zillion instances of "apparition" to "apparation".
Now, in HBP, JKR goes back to "apparItion" again, with nary a hint that it was ever spelled any other way.
As a reader, this doesn't really matter. As a fic writer, however -- *beats head on desk*
Speaking of fic, "If We Survive" has been Jo'd in so many ways that it's easier to declare it AU and have done with it than to try to revise it, so don't expect to see an HBP-compliant IWS any time soon. However, "The Potions Master's Apprentice" and "Personal Risks" are still reasonably undisturbed, I think, so I could just go back to that point and work from there, and I do think I may give that a try. Don't expect epic results, however. I have a short genfic involving Snape and Aberforth Dumbledore, and another (which could be a sequel to, or even part of, the first) with Snape and Maud meeting for the first time after Albus's death, but I think that's all I'm likely to get out of the can before the new baby comes...