Were there not americans prior to the signing of the consitution? Yes. If the vast majority wanted to further amend or even set aside the consitution, do you not think the people could do so?
So, if "No, society does not form its own morality" then exactly who does?

No, my dear igloo, societies do indeed set their own morality. Just like whats moral for say, in an African society, may be moral for them, but not for our society, is just the plain truth of the matter. Likewise, what may be moral for you and your wife to do in Vegas, may not be so moral to the Amish in Pennsylvania.
I am one who tends to believe that morality ultimately stems from religion, not vice versa.