By the way is this in anyway commissions related? I have done the mod a few times, and the core reason came down to either calculating commissions or trying to measure the performance of Sales People in taking and converting orders.
Keep in mind that the biggest issue, is that if the Navision order is not billed, then there is no record left to contain the “Original Ordered Qty”, which is why I have always found it necessary to create a new table.