This is a bit wrong. The actual producer of Star Wars and Empire Strikes back was Gary Kurtz. Kurtz quit for RotJ because Lucas was more interested in toy sales, as they make 3x the amount of money a movie does, than the actual plot of the movie.
Gary Kurtz interview.
Quote from Kurtz on the original idea for the ending of RotJ.
As for Empire, Lucas did change a number of items from the original screenplay that Leigh Brackett had written. Brackett died only a month after doing the screenplay for Lucas and in that original screenplay, Darth/Anakin was NOT Luke's father. So although most of Empire's screenplay was written by someone else, he did change it for several things.
Darth Vader was not originally Luke's father.