If you change your forum signature, it affects all of the posts which you have already contributed. So, yes, it *is* retroactive.
If you are using Signature A for awhile, and then you switch to Signature B, all of your posts will now display Signature B. However, it is possible that some of your older posts will still be indexed in the search engines with your original signature, and Google may not update this, especially if they are older posts on pages that have long been forgotten about.
Yes, All of you are correct, If you change the signature of your forum then all posted forum signature will change automatically. But If you want to index your previous post with new signature then you have to first index on Google then may be new back link will be consider.
By changing the you're forum signature, I think all your post will link back to your new signature. But I also agree cscgal, Google will be indexing your signature however some of it are still linking back to the older signature.
The point I was trying to make is that even though when you change your signature, ALL of your posts are affected, Google won't necessarily re-spider and reindex ALL of your posts. Therefore, you will end up with Google recognizing some of your posts with your old signature and some of your posts with your new signature.
Correct me if I'm wrong, but google doesn't "see" the signatures here on DW right? If you search for your signature for example, I only get 1 result back from Daniweb, which is your member profile. So all the "nice" SEO-fellas here, who are posting as much as the possibly can to promote their signature-link are actually wasting their time because it has no effect on searchengine results whatsoever.