You could duplicate the campaign, but choose a new character sheet for the campaign, but honestly, that's probably looking for problems. If you have a lot of time invested in maps, and since you aren't on a subscription level that allows the Transmogrifier, you could start with a duplicate for that purpose, but characters have so many attributes that are system specific, that there is a very good chance conflicts in naming will cause you more headaches than simply recreating the characters. So to sum up, I would recommend: 1) If you have a large investment in maps and tables, duplicate the campaign and delete all characters. Create the characters afresh. 2) If you have no significant investment in maps and tables, start over from the beginning.
