Merge-patch: only the fields present change. Nested objects (resendBehavior, settings) merge field by field, arrays (to, cc, types) are replaced whole, properties the API has no field for are never touched,
and an explicit null clears a field. triggerActionId and triggerActionType are immutable: sending their current value back is accepted, a different value is 422 immutable_field.
A blank templateId keeps the current template. A request that changes nothing returns 200 without writing or auditing.
Concurrent edits of the same trigger are last-write-wins, like the ControlUp Console.