Purchase Order - change status

Hi all,
   I'm trying to change the status of a PO. Orginally the PO is in status: Pending Bill and I want to change it to Billed, the status field is read onlly then I'm trying to change the PurchaseOrderItem. The PO is very simple, just has 1 line, I'm attaching the code:

private void markPOBilled(NetSuiteServiceManager port, String poID) throws Exception {
 
  PurchaseOrder po = new PurchaseOrder();
  po.setInternalId(poID);
 
  PurchaseOrderItemList poIL = new PurchaseOrderItemList();
  PurchaseOrderItem[] poItems = new PurchaseOrderItem[2];
 
  poItems[0] = new PurchaseOrderItem();
  RecordRef item0 = new RecordRef();
  item0.setInternalId("xxxx");
  poItems[0].setItem(item0);
  poItems[0].setQuantityBilled(new Double(1));
  poItems[0].setLine(new Long(1));
  poItems[0].setIsBillable(Boolean.TRUE);