They cannot change the voices of the Stormtroopers because of one simple reason why?!?
If they did, when Luke and Han were dressed up as Troops, as soon as they spoke, they'd get shot. The only 2 Stormtroopers in the last twenty years to sound different.
So no way can Lucas or anyone else do a thing about the OT Troopers voice. I think it's gonna stay the same!
Oh and also, I remember having a magazine poster thing in the 80's, about ESB time, where it said on the back that Stormtroopers were clones.