Uhh.. wrong. (I don't usually post, but I had to for this one.. )
Chester's voice did not change... I don't understand why people don't get it.
Before 2004 he used a different technique, his voice is the exact same.
Ex. Numb
-Listen to LIT 2003. The verse was sung in his regular 'voice' and the chorus he uses this throaty technique that Don Gilmore mentioned in an interview - he would sing with 3 pitches, in turn, making it sound 'raspy', but Chester never had a raspy voice. When he did, the 3 pitches together is basically like 'screaming underneath with a pitch on top'.
-In RTR 2008, the verse is the same (except not as youthful, but THAT'S age) but the chorus was sung in his regular voice, not with the throaty technique.
I agree that he lost some of his power in his screams and his voice doesn't sound as strong, but the loss of the 'raspy voice' that was never there isn't true. He just never had a raspy voice.
In 2001, his 'raspy' voice sounded different because he didn't master that technique yet (it's also really stressful on the voice so that's probably why he stopped).