From the sound of it, Chris Terrio is not rewriting the movie from scratch, but simply going over the David Goyer script to put the finishing touches on it, or as the source describes it "get the script in shape."
Terrio has a good working relationship with star Ben Affleck and one wonders if Terrio was brought in as part of the deal to get Ben Affleck. Perhaps the star wants to avoid another Daredevil debacle by having someone he trusts go over the script. Though Goyer is also not available, as he has moved onto other projects, including Sandman, so maybe I'm overthinking it.
Aside from a couple exterior scenes that have been shot so far, principle filming is not expected to begin until early 2014.