PO Change Order (Amendment)

After a PO has been issued, a buyer raises a change order to modify quantity, unit price, delivery date, or delivery address on one or more lines. The system determines whether the change requires re-approval based on configurable rules (e.g., price increase > 5% or value increase > $1,000 requires re-approval). If approval is required, the change order enters the approval workflow;
on approval the PO is revised, the amendment number is incremented, the supplier is notified of the revised PO, and the budget commitment delta is updated atomically.

po_change_order { id: string, po_id: string, amendment_number: int, status: enum, requires_approval: bool, submitted_at: timestamp, approved_at: timestamp, external_id: string };
po_version { id: string, po_id: string, version: int, snapshot: jsonb, actor_id: string, changed_at: timestamp };
budget_commitment_delta { id: string, change_order_id: string, delta_minor: int64, currency_code: char(3), applied_at: timestamp };
(reference, product may differ).

POST /v1/purchase-orders/{id}/change-orders { lines[{po_line_id, new_quantity, new_unit_price_minor, currency_code, new_delivery_date, new_address}], justification } -> 201 { change_order_id, amendment_number, requires_approval };
POST /v1/purchase-orders/{id}/change-orders/{co_id}/approve -> 200;
GET /v1/purchase-orders/{id}/versions;
emits procurement.change_order_issued, procurement.supplier_notified events;
idempotent via external_id.

DRAFT -> PENDING_APPROVAL -> APPROVED -> TRANSMITTED;
terminal REJECTED, CANCELLED;
guard: PO change blocked after all lines fully received and invoiced;
cancelled lines release budget commitment immediately;
budget delta ACID with version increment.