Making of ROTJ reveals that Kurtz was indeed replaced during post-production on ESB (in late 1979). According to Howard Kazanjian's recollection, Kurtz showed up one day and Kazanjian had to tell him that he was no longer allowed to work on George's movie.
Which came as a total surprise to Kurtz, apparently. Kazanjian had expected that Lucas would have told Kurtz he was being let go, but apparently Lucas preferred to let Kazanjian do his dirty work for him. (Later, when asked by Kazanjian, Lucas would deny that he had failed to inform Kurtz of his impending dismissal.)
Kurtz accordingly sent in his letter of resignation to Lucasfilm on December 11, 1979.