One of the Badassdigest guys is saying that Abrams simply didn't like Arndt's script, and yes, it's like a total do-over. This is how/why the rumors started that Abrams was thinking of backing out. The truth was that he wanted to do it, but was unhappy with the story. The other big takeaway here: no way is Summer 2015 happening.
Source here:
https://twitter.com/devincf