The changes in the 3rd draft's revision are not sweeping by any means, but they are important. It's not only a chance to make sure that events (and locations) in the early chapters are in continuity with the final ones, but is another chance to correct some mistakes in the storytelling which I can either chalk up to lack of foresight, or laziness! It's the type of thing that can only be done with the entire story written, the sprinkling of details and tiny events which serve to tie the entire work together.
At any rate, I just finished with Book 1, though I may return to it before the 3rd draft is done. I except that it's Book 1 that required the most attention, but I will go through the entire story with the same care nonetheless.