All user's posts should show the latest signature, because signatures are independent of posts. If the user had no signature when the post was written (or unchecked the "Show Your Signature" box), then you will have to run a query on the database to update old posts to set the flag for that particular user. I will see if I can dig you up something.
This will toggle all post that do not have the signature flag set. (Global)
Code:
UPDATE post SET showsignature = 1 WHERE showsignature = 0