Welcome to the Lithosphere.
You ask a good question. However, the answer somewhat pivots on whether your community is Single-Sign-On or not. Is your community using SSO?
We use SSO.
In our production environment our users are able to change their own login name.
However, in our staging environment, we recently changed the config to read a username from the login cookie, and we cannot get the UI to change username to show, even after giving users the permission.
We did not have the permission set up properly in our staging environment before, so we don't know if it would have worked before this config change either.
