More generally, as far as redubbing a major character, Grievous is a good choice due to not needing any lip sync. I’d be curious to read your version of his dialogue incorporated into the scenes of the film.
Well, here’s my idea for the first couple of Grievous scenes. I attempted to do three things here: convey Grievous’s new history with Obi-Wan, imply that he’s at least somewhat Force sensitive, and reduce the Jedi’s flippant attitude towards him (since he’s no longer a joke character).
Talking to the Trade Fed captain
GRIEVOUS: I sensed an intruder, captain.
CAPTAIN: Two Jedi have landed in the main hangar bay. We’re tracking them.
GRIEVOUS: There you are, Obi-Wan…
Meeting Anakin and Obi-Wan
GRIEVOUS: Ah, Master Kenobi! It’s good to see you again. If you’re here to apologize, it’s too late. And… Anakin Skywalker. Do you really replace your apprentices so easily, Obi-Wan?
Grievous walks away, and turns around, revealing his saber collection.
GRIEVOUS: Your lightsaber will make for a nice memento, master.
OBI-WAN: Not this time. And this time, you won’t escape.
Obi-Wan and Anakin break free, slashing up battle droids until they finally confront Grievous.
GRIEVOUS: This isn’t the end, Obi-Wan.
Grievous breaks through a window, uses a grappling hook to grab onto the hull, and ejects in an escape pod.
GRIEVOUS: We’ll meet again.
The more I think about it, the more I’m leaning towards just keeping Grievous as is (and either keeping Anakin becoming Vader or doing the Adywan “make Vader seem like a generic mook” thing), but I’d still like to see if I can make this work.