I discovered a workaround that works perfectly, to resend an existing invoice to a different email address while keeping original invoice #!
Log in and go to your "Manage invoices" tab. Click the checkbox next to your invoice you want to resend, then (under the "Action" column, click "Remind". This will take you to this screen:
"Reminder… we'll send this email to remind your recipient that payment is due for this invoice." And it shows the existing email address, etc. Then you can select one of 2 buttons: "Send reminder" OR "Go to invoice details".
So here you must select "Go to invoice details", and the next screen will allow you to change many of the invoice details, including the recipient email address, while still keeping the same invoice number.
I did this and it worked perfectly. My client received the invoice at the new email address and paid me immediately.
