That is one of those odd numbers thrown out there. 50% of what, the wording of dialogue maybe....but they already said story and plot were "on lockdown" which means no scenes were changed or likely none were added (since the story is a breakdown of the plot). Apparently is 50% was changed than the other 50% is what showed up in the teaser, in film diaries and in shooting schedules released to the public...because those match "Prime Directive". They changed it during shooting if that gives you any perspective on how much it changed. That means after they okayed the script, booked the locations, made the props, storyboarded the scenes and got coffee and breakfast for the people involved they started rewriting things. If anything expect better dialogue...they'll probably cut out some of the cornier lines, make the jokes have more punch (as they promised). X3 apparently changed a bunch too, so did Fantastic Four...to bad those turned out to be gross exaggerations. It's called damage control, if people don't like the script...they still have to get them comfortable enough to watch the movie or they lose money.