Hello @ShangY ,

i am having an issue when i follow these steps. I have the same position as the original poster of this thread. I have an estimate that was approved, and have 2 paid invoices and 1 unpaid invoice against that estimate. 

When i follow your steps and select the project from the customer dropdown, all of the fields in the invoice are cleared. 

How do I change the customer to the project without having to recreate the invoice?