It's going to depend, I think. Jim has said two somewhat contradictory things. First that it's going to be goatees and eyepatches, which suggests that it's a Star Trek mirror universe where everyone is radically different. Like the episodes of the show. But he's also talked about the Choice Harry makes, which suggests that he's the primary focus. And as Michael has said, if Harry goes bad, Michael will be there to stop him. Likewise many of the other characters would not change that much. Depending on what and how, we may see changes to Thomas, Murphy, the Alphas, maybe Eb, to make them different from the kind of people we know, but others, say, Morgan, the Merlin, probably Mab, would be the kind of people we already know, with the difference being how they react to Harry.