If you go to someone's profile there is a small icon next to their name that shows the previous usernames. Out of respect you should probably not mention the name outright, but it should keep you informed if that's what you wanted
leo @'d me in a thread about his last dream, stating he was passionately making out with a guy he used to know and then woke up while reaching for his crotch, and said that i would "know who 'tf' it is"
and of course i know, it's luke