You would need the Sales Order Number which is the key to the sales order file, otherwise it will create a new key and hence a new sales order

Thanks for your reply.

Is there a way for Visual Integrator to find a sales order number from a PO?  When I run the importer, it gives a warning concerning duplicate PO's so I think there should be some way to do this within Visual Integrator itself.

An easy way would be to use Microsoft Access, import or link in the Sales Order Header file and import or link in your source file and create a query and link on purchase order number and build a new source file and use that file to in your import.

That warning is an SO Option to find duplicate customer PO and not a feature of VI.  If you want to update an SO by VI, you need to feed that SO number into VI. 

Using an MS Access query (like BigLouie suggests) is one option.  An Excel query and VLOOKUP is another.  I've also scripted an ODBC lookup to edit a CSV with such a value as an extra column.

If you want to pay a master developer / providex programmer to do something with Perform logic, that might be feasible, but those kinds of things can be expensive.