Changing the preferred vendor of an item on a sales order.

Hi

I'm trying to alter the preferred vendor of an item after it has been added to a sales order so that it will create a purchase order with the cheapest supplier rather than whatever the item's current preferred vendor is.  The process runs over the sales orders in Netsuite, collects any that do not have purchase orders and works out the cheapest supplier for the products on the sales order.  It then updates the preferred supplier on the item, saves the item and then updates the sales order item line's "Create PO" flag.  This stimulates Netsuite to create a PO