LFL's own press release has Kennedy saying they're very happy with Arndt's work ("I am very excited about the story we have in place and thrilled to have Larry and J.J. working on the script,". I don't think she's saying that to save face, so close to filming it has to be true or Arndt would have been fired months ago. They're may be disagreements underneath or irreconcilable differences in details of the writing between Abrams and Arndt, or maybe Arndt wasn't able to deliver the fixes Abrams and the other producers felt necessary and he decided to step down and let Kasdan do them, we'll probably never know.

It doesn't sound like Arndt was fired anyway, more like they're in mutual agreement that he's brought the script as far as he could and they need Kasdan's SW expertise to polish it up and bring it to the final stage. Abrams's involvement suggests a lot there's something to be fixed/optimized in how the story written by Arndt translates into a SW movie (in the sense that Kasdan and Abrams will be working together and Abrams will have directorial input in the fixes asked of Kasdan).

She confirmed everything is under way and on the movie remains on schedule to be filmed next Spring (it was always to be March/April in England and not December/January as I thought, after all). The movie's development is probably 2/3 done or so at this point - it's the more concrete preparations (not longer designing but building costumes, props etc. transforming the designs into building plans) they're about to start on, they're certainly not "starting over" with the script in the sense of changing the story, though given it's six months and not 2-3 as I thought they could have decided to rewrite a part of the movie that didn't work as well as they hope, scrap scenes, approach a story line a bit differently in a more SW-ish way etc.. But essentially they have the "final story" for the essential of the movie or they wouldn't be that far in planning its production. At this stage they can well scrap scenes or add scenes, or restructure one or two story lines and stuff like this, but the core of the story is there. It's more likely the story telling style/tone they will be altering.

If I read right, Arndt has also written treatments for the other two movies.

The news would have been far more worrying had it been announced just a few months after Arndt started. That would have meant they had big problems getting the story/movie off the ground. So late it can only fixes, or refining/optimization.