I think it will change, the previous posts signature will change if you edit your sig to something else, the change will be reflected.
The members who are selling signatures, like he get 1k post, offer for sell, then in fact how is that solved? How can he sell that 1k if he cant edit?
He can change it because he has activated it before. You have never used it to will not be shown on your previous posts.
If you didn't have a signature till now, putting one up won't affect your previous posts. If you did have one and changed it, it would affect your previous posts.
I know thats why I have stated "the previous posts signature" if he is using for the first time, there won't be any previous posts signature.
the same thing happened to me earlier this week. i did not know about the signature rule good to see you figured it out. -dan