I was recently renamed from "E to Pi times i" to "Retro" on my Wikimedia login. I would like my Phabricator username to be updated likewise.

There should probably be a new "Modifying Phabricator accounts" column on the Phabricator workboard for things of this nature (it could also include previous Phabricator account deletion tasks and Phabricator permissions requests). Previous requests have been lumped into "Administration (UI)", but that seems a bit generic and misleading in certain cases. I count about 20 previous rename requests and probably about 40 total previous tasks that could be classified in this way.

Alternatively, I guess there could be a subtask for such requests, but I'm not sure if that has any advantages over a new workboard column.

And I am aware of the changing username Phabricator dialog:

The old username will no longer be tied to the user, so anything which uses it (like old commit messages) will no longer associate correctly. (And, if you give a user a username which some other user used to have, username lookups will begin returning the wrong user.)
 
It is generally safe to rename newly created users (and test users and so on), but less safe to rename established users and unsafe to reissue a username.
 
Users who rely on password authentication will need to reset their password after their username is changed (their username is part of the salt in the password hash).
 
The user will receive an email notifying them that you changed their username, with instructions for logging in and resetting their password if necessary.

My username is not in any previous commit messages and my Phabricator account is managed through OAuth, so hopefully this should be straightforward.