There's no way to batch force-print transactions that aren't marked to print.
But you can print them individually by opening each one and clicking the Print button at the top. There is no need to mark them to print later first.
Still, are you sure you need to to this, to actually print them in invoice form? A much much easier option is to create a Journal report, which shows all the accounting details for each invoice. You can filter it for TransactionType = Invoice, the desired date range, and then add typical data columns you see on an invoice that aren't there by default, such as the Item, Qty, and Rate.
Start at Reports | Accountant & Taxes | Journal to create the report.
