Changing sales orders from cash sale to invoice in bulk?

Our process:

- we upload a large .csv file every month for mass billing to customer credit cards
- we choose a "sales order -> cash sale" custom form
- we then bulk bill the cash sales to credit cards on file
- but usually some of these billings fail - the credit cards decline

What we want to do:

- bulk convert those unbilled sales orders from "sales order -> cash sale" to "sales order -> invoice"
- bill the sales orders into invoices
- mass email the customers that they have a past due balance (send them statements)

The problem:

- currently we manually convert each unbilled sales orders so that it bills as an invoice