First thing’s first: according to THR, the script for Star Wars: Episode VII has been significantly reworked since Abrams and Lawrence Kasdan took over screenwriting duties last fall. The official reason given for the screenwriter change was that the Episode VII crew was facing an impending deadline with the film’s December 2015 release date, but THR says the actual reason for the shift was a difference in opinion over which characters to emphasize.
Arndt apparently felt that the main characters of Star Wars: Episode VII should be the offspring of Luke Skywalker (Hamill), Han Solo (Ford), and Princess Leia (Fisher), with the original Star Wars actors taking supporting roles in the story. Abrams felt differently, as he was eager to focus Episode VII on Luke, Han, and Leia “so audiences could have one more chance to enjoy them before a fitting send-off.” George Lucas was also involved in the debate (Arndt wrote the treatment for Lucas before Lucasfilm was sold to Disney), but he deferred to Abrams and producer Kathleen Kennedy, and with Arndt now completely off the project, the lead characters of Episode VII will indeed be Luke, Han, and Leia while their offspring will take on supporting roles. The plan now is to have the younger characters take over as the leads in Episode VIII and Episode IX.