Time passes, things change. Me, you and that guy over that. We all change. Even Final Fantasy XIII.
Above is a screenshot Square Enix released in August 2008. Below looks like the same screen, but this was released in June 2009 along with the overseas version of Advent Children Complete.
The biggest difference graphic-wise appears to be the shoulders and shirt of foreground character Lighting.
Here is one of the first screenshots of pig-tailed Oerba Dia Vanille.
This is a recent screenshot of the character.
Compare this initial screenshot of Lighting (above) with a recent screenshot (below).
There appears to be less detail on Lighting's costume. However, the background certainly offers more mode than the earlier one. The hair textures seem different as well. Ch-ch-ch-ch-changes, indeed.
はちま起稿 - 「ファイナルファンタジー13」は順調に劣化しているようです [はちま起稿]