There are many factors on who should change or not, if they are a main character (knuckles,shadow,sonic,tails) then in order to keep modern, their voices will change more than the minor characters/one time apperance characters (vanilla,metal sonic,mighty).
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As of Sonic Colours, Sonic's new voice actor is Roger Craig Smith.
OMG my friend told me the other day that his voice changed and it's really bad, when I saw the commercial his voice WAS BAD, it was completely different, it was to deep! I think his voice changed because the last voice actor for sonic quite the occupation of voicing him (Which I wish he didn't!) IF that is what happened. Either that or SEGA where ready for a change. I don't like the change AT ALL though. They also got a new voice actress for Tails! Ridiculous, I know. They should have kept Andy Griffith (sonic) and Amy Palant (Tails)! I can never see Sonic and Tails the same way ever again! His voice is so different that it seems as though his personality should change with it, the new voices do not match the characters at all to me.
Sonic
SEGA decided that it was time for a change, the change may have also occurred do to the bad fan reception from Griffith's time as voice actor.
Amy Palant does the voice of Tails in the newer Sonic games and in the 4kids version of Sonic X.