They changed the voice because it wasn't Xeahnort's heartless, it was Riku, the difference in voice is used to show that.My biggest dissapointment was...
when they changed xeahnort's heartless voice
even though he only appeared for only a few scenes
anyway for the overall game, it was too easy and i don't think FF has a large enough involvement. although the disney characters are good (takes me back to being 7 again) it would be nice to explore FF a bit more, say midgar, spira etc.