The client is never changed. A temporary matter with entries on it cannot be deactivated: move the entries to the real matter first (409). narrative_language is an admin's to change (403 for billing), and not on a matter held by a connected practice system that supplies each matter's language, whose language is the system's (409); sending the value the matter already has changes nothing and is allowed. A matter the caller is not served by GET /v1/matters (a practice system that shows each person their own matters does not show it to them) is a 404, whatever their role.

Operator mount: an operator acting for the firm named in the path, as its oldest active admin, else its oldest active billing member; X-Six-Minute-Acting-As names them. 404 for no such firm, 409 urn:six-minute:problem:no-acting-member for one with neither. A patch naming attribute_sets alone is served for any firm; one that changes anything else (the name, the number, the description, active, narrative_language and the rest) is served for an internal tenant alone, and any other is 409 urn:six-minute:problem:tenant-not-internal, before the audit row and before any write.