UPDATE: Biggest Change in Return of the King? - Spiegel Online
The German online-magazine Spiegel Online interviewed Philip Capil (a.k.a. Beren), webmaster of the New Zealand Lord of the Rings web site Company of the Ring: Tol Galen. He was invited exclusively by Peter Jackson and Fran Walsh to the Edoras set last year and after his set-visit he asked if he could work at the set. They said yes! So he worked for six weeks as a production-assistent for Rick Porras at the bluescreen-stage.
Philip tells that everything he saw was very amazing, e.g. the balrog or Treebeard. And he was amazed at all the little details at the weapons and props.
Then he tells that the biggest change is at the end of the third movie, but he is not allowed to betray what.
Beren sent us the following e-mail to clarify what he said in the interview:
I said the biggest change was from book three, this is of the changes I know, but it might not be from the end of the book.