Yes, colmena, as usual, Roger is correct. There were many drafts of the screenplay. One example, which I quote extensively in my Epinions review, concerns the funeral of Kane's son, after a Fascist riot in Italy, in which Kane reads the epitaph on the tomb, an ancient, mordant poem about the foolishness of materialism. And of course, the most famous example of Welles' improvisation on the set of the picture involved his shutting down the shoot for a couple of days while, inspired by a play by his friend, Thornton Wilder, he came up with the montage sequence in which he summarized the marriage of Kane to Emily Norton Kane in a moment or so.
In many ways, the shooting continuity IS the screenplay for CITIZEN KANE.