According to StarWars.com, Lawrence Kasdan and Abrams having taken over the co-writing duties from Arndt, who has been attached since shortly after Disney's acquisition of LucasFilm. Kasdan has been a part of the Star Wars universe a long time and wrote The Empire Strikes Back, generally considered the best of the franchise. He's also been working with Simon Kinberg on the many spinoff films that are in the works.
If there were any problems to be found with the work Arndt had done, you'd never know it by what Lucasfilm President Kathleen Kennedy's words on the news...
"I am very excited about the story we have in place and thrilled to have Larry and J.J. working on the script. There are very few people who fundamentally understand the way a Star Wars story works like Larry, and it is nothing short of incredible to have him even more deeply involved in its return to the big screen. J.J. of course is an incredible storyteller in his own right. Michael Arndt has done a terrific job bringing us to this point and we have an amazing filmmaking and design team in place already prepping for production."
A script change isn't always a sign of disaster, and it may just be that Abrams and Kasdan have ideas they want to see implemented. Or they simply didn't like what it was Arndt may have turned in and called for an overhaul. Who knows? But at the very least this threatens to push things back a little back.
