Signature not retroactive?
 

So I have 40 posts without a signature. After the 40 posts, I decide I want a signature, so i make one. After I make one and make sure I show the signature, none of my old posts have been changed. But, if i make a new post it does change. Can anyone explain what Im doing wrong or what I need to do to get it working?

P.S I had made sure my signature was displayed even before I made a signature.