If a mod hasn’t dm you yet for username change then dm a mod for username change.
I dm’ed hugecat for my username change before.
Something discourse default setting or something.
I thought this had something about the why it’s request only, but it doesn’t:
https://discuss.eroscripts.com/t/how-to-change-username/81106
Another quick search for username reveals this short thread on the why, which is discourse setting:
https://discuss.eroscripts.com/t/is-there-any-way-to-change-username/3768
3 years since that, so maybe it’s possible for custom trust level users to change usernames now?