I've actually thought about this idea before, so thank you for making a post about it! I'd love to see this be implemented into the site.
Being one who changes their user constantly, I think this would be a neat idea. I've also had around 2-3 friends who have changed their user in the past month, so it'd definitly be helpful, or, the username change could have a limit or a timer. Something like, you can only change your username once or twice a year? Like the deviantArt system (although I know a lot of people who hate dA's username change system, but it's just an idea).
I'd like to see what the community and moderators/admins think of this.