this seems like a no brainer to me

can someone teach me how to modify my advanced guestbook so that when a reader makes a comment on an existing entry, that entry is moved back to the top as if it was a brand new entry. for example, exactly the same way this forum works. make a reply to a post 3 pages deep and that post moves all the way back to the top of the first page. it seems like this would be a good option from the admiin panel, if thats even possible. anyhow thanks in advance for any help.