So what required a redub, and what didn't with the flashbacks? That's what I want to know. Seems silly to mix old and new voices. I do think that George's current voice is actually quite similar to his original voice.
I'm guessing since older episodes are rarely seen anymore, they probably didn't want to confuse the little kids watching as to why their voices "suddenly" sound very different in the flashbacks.
Here's a comparison of "Traditional" and "Flash" Ratburn (from "Through the Looking Glasses" and "Get Smart", respectively). If anyone wants me to do this for other characters, let me know.
Well, I guess after teaching the exact same third grade class for sixteen years, Ratburn's just getting old: grayer looking hair, duller color in clothes, middle-aged spread...