If an invoice has been saved, it is no longer possible to change the customer on the invoice.

However, you can edit the contact information of an existing customer and update the information on a saved invoice by doing the following:

1. Go to the Customers tab

2. Edit the customer’s contact information

3. Save.

4. Now go to the invoice tab

5. Open the invoice

6. Click Edit 

7. Press Next and Save

Now the invoice recipient information is updated on the invoice.