Send only the fields you want to change; omitted fields keep their stored values. For example, {"is_enabled": false} pauses a monitor and leaves everything else as is. Only description accepts null, which clears it; null on any other field returns 422. group_by accepts at most one Dimension. System monitors accept only threshold and is_enabled. For what each cost_type value measures, see Cost Types.
Returns 404 if the monitor does not exist or belongs to another organization. Returns 409 if the change would give the monitor the same filter_set, group_by, threshold, and cost_type as another monitor; error.existing_id names that monitor. Paused monitors count as a match and deleted monitors don't. A change to only monitor_name, description, or is_enabled never returns 409.
CloudZero records the change in the activity log, attributed to "Set via API", after saving it. If that log write fails, the update still applies.