Eric Brown is our SS and I doubt anyone replaces him as the starter. Brown is more adept than McCree in run support and short routes against backs and TEs. McCree is more of a centerfielder and is better than Brown in deeper coverage.
Game situations will dictate the secondary arrangement. In week 1, I would expect more focus on containment of LT at or near the line of scrimmage. The Chargers have little in terms of multiple deep threats so you play Brown at SS. Week 2 is a different story. When you go against Charles Rogers, Roy Williams, Az-Hakim, and Tai Streets you want your more athletic guys in there and not necessarily the big hitter/run stuffer safety. That's where McCree will be helpful. Right now, Eric Brown's backups are the walking wounded (Ramon Walker and Earl) so depending on how they recover, any number of things could happen behind Brown.
McCree is more suited physically for FS so lets sit back and watch the battle between him and Coleman. However it all works out, we are adding quality depth in the secondary.