See https://twitter.com/BSD_Fenix/status/1174678474520485888, where a user with a good motivation to change their username wasn't immediately sure how to make the change.

This workflow has a couple of issues:

• There's out-of-date language about sessions. This language is no longer relevant after D20102 (Feb 2019).
• The dialog probably errs too far on the side of caution about username changes. When a user changes their human name and has a username based on that human name, a username change is completely appropriate. While the caution about reissuing usernames is warranted, the guidance is probably too cautious about username changes in general.
• When you aren't an administrator and click the link, it does nothing since the endpoint 403's. This is not consistent with how other UI elements work: clicking a disabled action should give you an explanation of why the action is disabled.
• We can allow administrators to provide guidance in Auth → Customize Messages so users are offered next steps (e.g., "email such-and-such for help").
• In the future, the "Quorum" workflow (T9515) might let users proactively suggest a username change that just needs to be approved, but this is some ways away.