Geez, I don't think it was done seamlessly at all, and it's something that's always annoyed me. He visibly retconned the scene in Lord of the Rings by saying that The Hobbit was written by Bilbo, and that the original version was a lie induced by the influence of the ring. That was seamless. But when he went back and changed the scene to the "real" version, that totally fucked everything up and was completely stupid and invalidated his own explanation. What was he thinking?!

EDIT: Oh, but in general, I take changing books just as seriously as I take changing movies. If I ever get my novel published, I will have it contractually say that the text can never be altered after the first printing, even if something as small a typo or two manages to sneak past the editor or some future book contradicts a plot point. Just leave it alone.