The funny thing is, they do not need to rewrite much. One way is to change the names and remove lines about those characters being Viconia and Sarevok. Another - they can be indeed imposters, or, better yet, assume the names in commemoration of the old characters (in a manner of "I take your name and every follower will think that you serve me").
I am sure there are dozens of other ways to do it without spending much money.