I think that in many ways FFXIII-2 is more of a change from FFXIII than FFX-2 was a change from FFX. The major change in FFX-2 was that the battle system was completely different. Aside from that, it was a very, very cheap reuse of almost all the assets in FFX. Entire areas, towns, fields, dungeons, etc were reused with some minor touch-ups here and there. Pretty much all the monsters were reused.
The reason FFX-2 did that, was because the actual FFX battle system wasn't very well received, but fans were mostly receptive towards the general area design and the way the world looked. They felt there were no major changes they needed to make to the approach to the environment and art, but felt fans would appreciate a return to an ATB driven battle system. They were right.
With FFXIII-2, the approach is the reverse. The most universally praised component of FFXIII was the battle system and the approach to combat. Most fans who liked the game at all agree that the battle system is the strongest point. The level design, the linear narrative, and the way areas were basically just pretty corridors were not so well received. The lack of interactivity in the environments was also not well received. So instead of changing the battle system, they have basically changed the entire world. Areas are all brand new, and they're adding more layers to all environments for exploration and extra content.