Take two...?

I imagine there must be many films where the original script has been changed as shooting has progressed. Indeed I remember reading somewhere that the script of "Casablanca" was pretty well rewritten day by day and not even the actors knew how it was going to finish.  But now we find that in the original script of Star Wars, Obi-Wan Kenobi was supposed to survive his light-saber fight with Darth Vader and escape with the rest of them.  This is courtesy of the bloke who played Chewbacca who still has a copy of the original script.  I wonder if the change was at the behest of Alec Guinness who made no secret of the fact that he disliked the film and his character and maybe wanted out?