Some fans have always insisted that Nobodies do actually have hearts, despite the games claiming the exact opposite. Enter Xemnas explaining they're half right. Nobodies are indeed created with no hearts, but develop replacements for their hearts over time.
Some fans have speculated that the depths of Sora's heart looks like the Destiny Islands. They were right, as when Riku dives into Sora's heart to wake him up, he finds Xion, Roxas, and Ventus on a perfect replica of the island Sora, Riku, and Kairi visited when they were younger.
Ben Diskin takes over Young Master Xehanort from David Gallagher, who voiced him in Birth by Sleep.
None of the Hunchback of Notre Dame actors reprise their roles except for Jason Alexander (Hugo) and Charles Kimbrough (Victor), though two of them are for good reason. The Tron characters have no reprisals period. although Fred Tatasciore Reprised his role as Kevin, and CLU from TRON: Evolution, and TRON: Uprising.
Xion and Kairi were voiced by Alyson Stoner for Days and Re:COM, possibly as a replacement for Hayden Panettiere. Hayden returns to her role as Kairi in this game, however, and has also replaced Alyson Stoner as Xion.
The Heartless and The Nobodies were replaced by the Dream Eaters.
Apparently, there was originally going to be greater differences between Sora and Riku's stories. Sora was planned to return to Space Paranoids, with only Riku visiting the Grid (there are elements of The Artifact here; note that Sora's plot is concerned with reconnecting with Tron, while Riku is the one with the condensed Tron: Legacy plot). Additionally, Sora's Symphony of Sorcery would've been Fantasia-based, while Riku's would've been Fantasia 2000-based.