Hi janetb,

Yes, there is. The @Regina_Lend_A_Hand_Accounting's answer is correct. You'll need to open the existing invoice.  Change the customer name to the project's name and click on Save and close.

This won't erase all line items and the invoice will be added in the project's transaction list. 

You're in the right place to get help with QuickBooks. Let us know if you need anything else.