They're 2-3 months away from filming (december, IRRC), in pre-production for a year. If things were not going well they'd have replaced Arndt long ago.

He must have done his script, pre-production is in full swing. The script probably could use tweaks and they got the veteran Kasdan, who was already acting consultant, to do them with Abrams. That's probably because the tweaks required fall under Kasdan's expertise more than Arndt's.

It's virtually the norm to bring in "script doctors"/"script editors" for the last phase of script writing (Lucas did the same for all the prequels, not always credited as often happens). It's writers' union rules that prevent the Studios from being too open about the work of "script doctors" - they like to preserve the prestige/illusion of single authorship - but Arndt was probably okay ($$$) with this being public and co-signing the script. LFL probably made this public because it's Kasdan who fixed/"rewrote" ESB and who then "wrote" ROTJ (he's done the script of Lucas's story treatment... him and a whole team that included the director, Lucas and others that participated in story development meetings before Kasdan took it all and turned it into a script) and they can capitalize on the fame of his name in SW fandom.

Pre-production/production design will have been in full swing for several months (in Lucas' days this started a year before he even finished the script), the production heads are hired and working, the locations scouted, the filming studios hired. Many larger parts are probably cast already (if they have original actors for big or small parts you can be sure they are secured by now), or casting or negotiations are ongoing (the bit parts not requiring many days of filming will typically be cast last unless they really wanted a big name for a cameo-like thing for whatever reason). If they followed the LFL method, which sounds likely with KK at the helm, they also have translated the whole script into 3D animatics by now, and Abrams has been busy for a while editing a rough cut of the movie from those animatics, with the most work focused on crucial action or trickier sequences of the movie, before he's even filmed it. That canvas will serve production as "storyboards" do for other movies, just far more precise. At this point they must have decided for several sequences/sets what will be built and what will be developed in 3D in postproduction. They'll incorporate into that version of the movie script revisions (if needed, depending on their nature) tweak things all the way until filming (and with SW, beyond that... they'll do some rewrites and re filming after the rough cut).

My bet, since Abrams will work with Kasdan, is that it's after the translation of Arndt's script to animatics that Abrams or the producers felt something still needed work and whatever it is (rhythm/tone etc.) it's probably in the "is this genuine SW enough? What could be tweaked to really get the SW tone" category, to which Kasdan himself, working with the director, is better suited.

Back in the day what Lucas liked of Kasdan was his understanding of the "matinée serials" vibe. He got the genre but was able to give a more believable twist to the "cartoon" characters, and he is better at humor and dialogues for this genre than Lucas himself.. (Lucas on his own tends to do it a little too close to the original cheesy serials). That's why he went to Kasdan to script Indy.

All that said I wouldn't expect too much that Kasdan's presence means something closer to ESB specifically. The shift in tone in ESB (that was "corrected" in ROTJ) was largely the work of Kershner that Lucas didn't like (Lucas was too busy with ILM to follow filming, when he saw the first rushes of ESB it was too late - it's mostly the tone he had the actors adopt and the direction of photography he found too modern and moody that GL didn't like) and reputedly neither was Kasdan satisfied with the departures/liberties Kershner had taken with the serials tone. They both said things at some point that meant they consider ROTJ was a return to form, in any case.

We'll see. I must say I don't have big expectations for the new SW. I'm sure they'll be at least OK and quite respectful of the originals, but they're not much of an "event" for me.

Edit: I should probably have gone looking for the full LFL press releases first. Kathleen Kennedy had pretty much confirmed all I've said regarding how pre-production is already reaching the final stretches and everything is booked for filming,with the principal artisans already announced and working for some months incl. set and costume designers, meaning they do have a "final script" and it's essentially revisions/tweaks/rewrites Kasdan is doing with Abrams. I'm sticking to my bet they have a good script but in the late phase of pre-prod they're finding it could use an extra dose of the SW-vibe they're counting on Kasdan to bring.