I don't think anyone at anytime said that Lost was 100% planned from start to finish. That is impossible when doing a show that has to work with live actors and network production. THe core structure of the story is what they had planned with little things tweaked here and there.
If you go back to the earliest episode where Locke is showing Walt the game explaining that it was over 2000 years old and served as a metaphor of good and evil. That plus the Adam and Eve skeletons are "proof" of this plan.
