Is your character basically the same still (same name, same family, other parts of background still the same, etc.), then you continue the thread. If it's a completely new character (with new name, family, background, etc.), then open a new thread but please leave me a note that the old one can be locked and/or graved.
If you keep the current thread: If you just edit the original post, it's unlikely someone will notice it if there's no new post to read. I suggest you post the new introduction in a new post, edit the original post with a note that the character has undergone dramatic changes and that a new introduction can be found in post #[?].
Hope that helps.