hence the name Versus, as in a kind of opposite or opposition to what FFXIII is, this was what they said when choosing the name but it was more for the world difference and the darker style to everything but the battle system change could work for the name.
sounds sorta like valkyria chronicles..............awesom e
If being different justify a name change then Versus Xiii needs a name change as well, since the battle system, characters, etc are very different.