How to implement Billing Mid Month

We were using the ERP till now upto the Delivery note stage that SO---->DN but now I would like to make invoices as well but the problem is that the past DN which have not been billed would still show up as unbilled, is there a way to actually nake these DN not appear as unbilled.

Also suppose we implement billing for 3~4 months but don’t track payments and then if we want to implement tracking payments then how could we do the same with unpaid invoices.

Please note that I would not like to delete these DNs or Invoices as they would be needed for future references.

Also please elaborate about the ARCHIVE button in the ERP as there is no documentation on the same.