Though, if Versus was in fact renamed to XV then that would mean that the entire project suffered a massive change at one point that made Versus no longer part of the Fabula Nova series like it was originally planned.
No, it really doesn't mean that at all. Type-0 was also renamed (remember Agito?), but the mythology of the world stayed the same. Nothing about the game
changed. It's just a name.
Honestly, renaming Versus to FFXV makes more sense at this point. The connections between the games were always pretty weak, and then consider the fact that XIII got two full-blown sequels. The average person might see the XIII in the name and think that it was yet another sequel of some sort, when it's not and never was.
That and the fact that Versus XIII was always a stupid name...