Problem: I need to change the number of an invoice, but the Invoice No. field is inactive.
You can’t change the number of an invoice for which a credit note was issued.
Solution:
1. Make sure the invoice status is Not Ready (the details of the invoices in Ready or Sent status can’t be changed).
To revert the invoice’s status, open the invoice page in edit mode and click the 
arrow in the Invoice Status field.
Switching statuses for Sent invoices may be forbidden in the Configuration menu > Clients and Vendors > Client Settings > Invoicing tab.
If the invoice is in the Sent status, don’t forget to resend the invoice to your client after you change the invoice number. To do so, revert the status back to Ready and click the Send by E-mail or Edit and Send buttons that appear in the Type section.
Invoice numbers are assigned according to a numbering scheme. To change the default numbering scheme, go to the Configuration menu > Templates > Numbering Schemes. Here you can edit the Client invoice numbering scheme or add a new one of the same type. To learn how to do that, see the Templates - Numbering Schemes article.
2. Make sure the Invoice Number Modification feature is enabled.
To do so, perform the following steps:
Go to the
Configuration menu > Clients and Vendors > Client Settings > Invoicing tab.
Check the Enable Invoice Number Modification box.
Click the Save button.
Now you can go back to the invoice in question and change the invoice number.
