On our survival server, players who have been on the server before the name changes are seeing the login message "NewUsername (formerly known as OldUsername) has joined the game" past the first time logging in after the changes. To our understanding it was supposed to be just once and then any future logins would show the new username as normal. My own account, Raven(Ravens_Requiem) as well as my friends, Othello(Wolfes_Howl) and Eyrie(Eyrie_the_Eagle) have witnessed this first hand.