I am not a lawyer and I would find out what is their definition of redefinition.
I might say you could, because you are not redefining the demagogue TEMPLATE. You are making a demagogue CLASS. It is a sticky issue and I would not seek to push those boundaries until you see what becomes of the GSL in the real world, rather then a lot of the hypotheticals being posted.
With that said, I would simply make a character class and call it "fanatic demagogue" or "demagogue priest" or some other type of adjective to your demagogue class name. That way, you are safe from conjecture.