account names cannot be changed.
you can only change your display name, which is available in the settings.
whether display names or usernames are shown depends on the interface/client and user settings where available.
the only way to change the username is to create a new account.
this isn't true. it was incorrectly stated in the upgrade guide but has been removed a while ago. it was supposed to be a recommendation due to some issues with postgres 15. there is no postgres upgrade required between 0.19 releases.