Automatically sets the invoice number as soon as an order as it's placed with a complete, processing status or always.

Install extension and it will add a new event to be called before: catalog/model/checkout/order/addOrderHistory/before.

Also the 'Invoice Prefix' can be updated automatically based on the date order is placed. E.g from store settings set value 'INV-{YEAR}-{MONTH}-' which will generate invoice number: INV-2016-12-11