Consider allowing users to change their username

Would probably want this on some sort of cooldown, maybe once a month. But it's also really niche so probably not worth doing

No files, tests, or entry points are identified. Start by locating the existing username and account-management flow, then clarify whether username changes are in scope and what cooldown and validation rules apply. Done would require an agreed behavior and corresponding implementation and tests.

Written by the indexing model from the issue text.