Even with that, with its famous 5-year series bible and whatever, JMS admitted that he made changes as he went along. From a 1994 Usenet post:
We looked at the story, the way a novelist looks at a novel he's one-fifth through writing, and you learn things writing the novel that you can learn NO other way, however much you plan and outline. And at some point, you say, "If we leave things JUST as they are, no changes, we can do X and Y, which are both very cool. But if we take a chance, and make some shifts in direction, we can do X, and Y...and Z...and Z is very interesting indeed.
(That page is all about him saying "But this won't affect the story's outcome", but the indisputable facts are that he didn't end up telling the story in the way that he originally planned.)