Cleanest way to change billing method after an invoice has been created?

Cleanest way to change billing method after an invoice has been created?

Hello,

I've added a task with a different rate/hour to a project but cannot change the billing method.  I get the following error method: 
  • An invoice has been created for this project. Hence, the billing method for the project cannot be changed.

Do I have to create an entirely new project with the new billing method (based on task hours)?  That seems cumbersome.

Thanks in advance.