@PASC 

Don't change older invoices to do this.

Instead, interest can be added to the customer's balance using finance charges (which are really invoices) and the finance charge feature built into QuickBooks.

The finance charge feature includes the option to either charge finance charges on just the original invoices or to include past finance charges in the calculation - which is what you want. The option to calculate finance charges including past finance charges is essentially compound interest - interest charged on the prior interest.