Run 2 separate and different imports on the same data set?
I think that you'll need to import into a temporary table in your file. And then do two separate imports from the temporary table.
One you have imported into the temp table, perform a find that isolates the records for existing invoices. Import them with your Import matching and limited field mapping, then perform a find or use Show Omitted Only to set up a found set of the new invoices and do a second import to add the new records.
Once you have successfully imported both sets of records from the Temp table, you can use Delete All Records to empty out the temp table.
I already have something sililular to that to ensure the my customer and vendor id are accurate...
Ill modify that to accomidate this as well.