Import Rules

Can you bulk export invoice details make changes and reimport and the changes will overwrite existing data. I defined my customer account terms and mistakenly though that would define the invoice terms once we imported all the historical data. Have since found that the the invoices are all due on the date created and the system ignored the customer terms. I cannot seem to bulk fix this without going into every entry individually. The reimport skips the records as they already exist. Please help