vaderios said:
GL stated that he grievous should NOT look like vader.

Plus what you propose is near impossible to be done.

-Angel

That's part of the problem. The PT is exactly what George wants as it is.

As for changing it sure it would be very, very difficult but I imagine not impossible.

Grievous is a model (a CGI model but a model none the less).

Some of the shots of Christopher Lee mapped over his stunt double were pulled off well (not all sadly) and that was one human head on the body of another.

The mask I linked to will soon be available to buy, film and scan and it's a model that can be mapped onto another model, surely easier to pull off than mapping a human head onto another human body.

The handling of the Vader's identity was really buggered up by Lucas so throwing in a bit of ambiguity serves the story better.

If I had my own way Grievous would look almost exactly like Vader so when Vader turns up in ANH first time viewers would just assume he was brought into the Empire as part of the peace process.

It seemed daft to me that Lucas wrote in a cyborg general and didn't try to pull something like that off.