The helmet for Episode III was completely remodelled.The helmets in the OT were all derived from the one source however they also changed extensively between films most noticably between ANH and ESB.The costume itself was not that different other than the sizing had to be different to accomodate Hayden.The helmet is a source of much continuing controversy.Personally I really like the design for the vader helmet for Epsiode III.The main changes were the widened back,neck flare,flat lenses as opposed to curved for the eyes,sharpened cheeks and the neck piece and face plate were one piece.The main changes that are immediately apparent to look at are the neck flare and the lenses.
I agree that the differences were to make him look newly constructed and that the look of the helmets between ROTS and ANH would make for a process of de-evolution rather than evolution due to damage and reconstruction,etc.
Here is my Master Replicas Episode III Vader helmet.The MR was taken from the original Episode III mold so this is entirely accurate to the last detail as to that which Hayden wore on screen:
