Sluggo said:
We had a rather lengthy discussion on the Maul/Grevious idea a few years ago. A lot of folks made a good case for it, and a lot made a good case against it. I think the concept was ultimately discarded as being impossible to pull off.

Take a look around the old discussions in SW Preservation and General SW Discussion and see if you can find them.

It needn't be that difficult, Adywan removed the red eye lens from every shot of Vader and changed the colour of Artoo's lights in ANH:R (not that I'm down playing how much effort went into that but is show's that it's possible).

Sticking Darth Maul eyes on Grevious would be only slightly more taxing, you could construct a moment of realisation during the Grevious/Obi Wan duel by dubbing a new line in, adding an eye close up and maybe a very brief flashback and Bingo!

Grevious used to be Maul and his sabre prowess and hatred of the Jedi suddenly make sense.