Art Jones was originally on the job but he has been replaced by Nancy Richardson, who edited the original Twilight film.  Summit isn't blaming Jones, only saying that they needed a stronger edit.  They are calling the material they've gotten from director David Slade "very good."

Slade and Jones had previously worked together on horror flick 30 Days of Night, and Hard Candy with Ellen Page. 

My inside sources (the voices in my head) tell me Jones was replaced with Richardson because he didn't have enough shirtless teenagers, or pouting in his cut of the film, those being most of what the movie is about from my understanding