You can change the invoice number by adding up to five symbols to the end of the number
On the Invoices page, in the Action column, click the Select button, and then click Change Invoice#.
In the Invoice dialog box, enter the appropriate symbols in the box, and then click Change.
The invoice number will be changed accordingly.
Important! Adding characters to invoices may cause an issue when you export paid invoices into QuickBooks as any additional characters beyond the 11 limit for the 'InvoiceNumber' field will not be accepted.