For one thing, Haley Joel Osment's voice changed waaay back when he was doing Secondhand Lions (he squeaks quite a bit xD) Once your voice changes, it really doesn't change at all after that. So the KH2 Sora is pretty much the voice we'll be hearing for the rest of the series, which hopefully will continue. *crosses fingers* The directors may ask him to change his voice to sound a bit "older" in the next installment, which he'll be able to do, because this is what he does for a living.
Look at David Gallagher--he was about 16 or 17 when he was doing the lines for KH1, playing a 15 year old. His voice had already changed by then--so he didn't sound at all different in Kingdom Hearts 2, except I think they asked him to go for a lower and more mature pitch than in the first one.
That's the thing about voice acting--they're professionals. They will tweak their voice to however they need it to sound (they've done so many lines for Kingdom Hearts, they can probably call up their "Sora", "Riku", and "Kairi" voices whenever they need to xD). And if it's not working, computers are doing amazing stuff with voices these days. They can make people who sing in a monotone sound awesome--and they can do the same with simple, speaking voice pitches.
I don't think anyone needs to worry about the characters sounding the same. Everyone who's really important already had their voice change, and it's going to stay the same for a while now.
*crosses fingers for a KH3*
His name is Haley Joel Osment.