I heard a little more from some insiders as to why Edgar Wright is no longer associated with Ant-Man, despite it being his baby for countless years.
He actually tried to start this project way before Marvel officially took off with the various phases. Like before Iron Man 1 even. He wrote the script a long time ago with the movie being a stand alone entity, and was unwilling to change it to fit into the "Marvel Cinematic Universe" so that Ant-Man could be tied into the Avengers at some point down the line. He wanted a solo project and that's all. He wouldn't budge from that.
Marvel put their foot down and demanded it became part of the universe instead of a stand alone. I mean, every movie they do (Marvel based) has had the goal of becoming much more. He didn't like their demand, so decided to leave due to difference of opinion.
I'm sure they are going to keep the script almost completely the same, but add in a few things that will allow this movie to tie into the Marvel Universe as a whole. This way it all makes sense down the line.
For this reason I agree with Marvel in saying bye bye to Edgar Wright. Yes, he may be greatness, but you need to work with the team. There's a much bigger picture, and they plan on that picture being that they're putting up so much $$ for each film.