How do I change an invoice I've already sent into a pro forma, which is what my customer wanted?

Hello Hello180, 

Welcome to the Community page, 

You can create a custom template and have it labelled as pro forma then just select them template before you sent the invoice off to the customer. 

If you go to the cog wheel>custom form styles> new style>invoice>design>in the first box left hand side rename it pro forma >done 

Then when you have created an invoice before sending it click on customise at the bottom middle of your page and make sure the pro forma one is ticked then you can save and send it to the customer.