The JustMarried plugin can be used to rename existing users. However, when a user is renamed, the alias used in the roster of all other users remains unchanged.

An option should be added to change the roster alias. I don't believe that there's a way to apply that change to rosters that do not live on the same server instance (for example, roster subscriptions though server federation won't be update), but we can apply the change at the very least for all local users.