Apparently your sigline is no longer stored with your post. When the software reconstructs your post it pulls your current sig from your profile.
So when you change your sig you change it for all posts, new and old.
As someone (I forget who) pointed out, though, think of the possibilities!
You could make yourself look like Nostradamus. Change your signature to say, e.g. “A virus program will travel the world on 4 May 2000”, and then display an old post of yours to convince people that you can predict the future.
I noticed that feature- it totally killed all suspense in Aha’s “Hello Mr. Jdavis” thread.
Not that I was on the edge of my seat wondering if he would be welcomed…