The entire bit...
Those who have seen all six movies now know that there is a direct link between the Clone Troopers of the first three episodes and the Storm Troopers of the three that follow. So are all those soldiers from the original trilogy actually just the same guy? “Yes,” Lucas said. Fair enough, master — but if that’s true, then why do the troopers in the earlier movies possess different voices?
“Well, yeah, some of the voices have been changed, um, to make it more consistent,” he said, raising an eyebrow at the precociousness of the Padawan. Indeed, Lucas has gone back and changed the voices of the Storm Troopers during recent re-edits for the special editions. “Those particular two guys that you’re talking about — which I know about, which is a very good performance — the idea is that over time, there were new clone strains introduced, and then they even conscripted guys to be Storm Troopers. So it’s not just purely clones: It started out as clones, but then it got diluted over the years as they found out they could shanghai guys [more cheaply] than they could build clones.”
-Uncle George