I think that in all honesty, Mandy did a pretty good job of Aeris. I was expecting her to be crap, but she was a lot better than I thought she would be. Actually all the voice actors from KH were very good, much better than those from FFX.
I would love to hear them done over with voices, it would certainly add a new depth to the game. Although the original will always be the best
Barret's voice over could be done by Mr T ^_^