It would be great to have a display name as well as the login name - was asked the other night by one of my testers if he could have a different name. If there was a display name that the users could edit I could say 'yes, do it yourself'. Instead I have to say 'I will create a new user and you have to upload your images again'.
Currently not a feature in coppermine. Use bridging if you need that feature badly. However, why don't you (as admin) just change the name of the user instead of making him register another user account and re-upload everything?