It would be a huge thing, to remove Granny from the story - however, that's what Granny is all about: The Story. She says, "The story goes on..." regardless of the characters or the time and place. It's always about the story. To remove Granny and put in a new Hag O'Hags (Tiffany may be a Chalk witch tied to the Chalk, but Granny reminds us that the Chalk also contains flint - and to those ends, Granny is entirely out of her element when she's not in Lancre) would illustrate that principle - that the story always goes on.