Usernames change. If your username can't change, you designed your system wrong.
Systems that are heavily persisted-comment-based at least have the excuse where changing a username would invalidate old comments referencing that user, but even GitHub still lets you change your username (if you link @-references to users, please store the user's ID in the backend text and only convert it back to a username when displaying; GitHub doesn't take this step but it should).
P.S. Hacker News, I'm looking at you.