J.J. Abrams clarified the reason for replacing Michael Arndt with Lawrence Kasdan and himself on the Star Wars Episode VII script: it was about timing. "It became clear that given the time frame and given the process and the way the thing was going that working with Larry in this way was going to get us where we need to be and when we needed to be," Abrams told Deadline. The comments came during a media conference call for the upcoming Almost Human TV series on which Abrams serves as executive producer.
Abrams also called Arndt "one of the best writers around" and said that his departure "doesn’t preclude working with Michael again in the future" -- whether on another Abrams-helmed Star Wars film or a different project, he didn't specify.
Kasdan, Abrams Take On Star Wars Episode VII Screenwriting Duties