Supposedly, according to King’s intro, the additions were things that he had to cut from what would have been his final draft purely because his publisher had a page limit. His actual changes to the text beyond adding things had to do with updating the story to include references to things like the AIDS epidemic. If I love the book I may track down an original version. I don’t think it’s sold new anywhere, although all copies of the Stand are still advertised as “Uncut” so it isn’t like the Star Wars situation where we got a replacement.

EDIT: I think he changed the year from 1985 to 1990 as well.