Personally I really think this was done to prevent the cat's face from being covered by the larger ESRB logo. It would have been silly art direction to allow one of the main characters on the boxart to have half their face obscured by a logo. Besides, they didn't need to move her that much to hide the cleavage, but they did move her enough to mostly get her out of the way of the logo without her covering up too much of the rest of the art. The cleavage was just a casualty
Edit: Also, if you compare the placement of the other characters, they've been moved around a bit as well. Again, probably to accomodate the placement of labels. The original Japanese version might not have even been a finalized design.