Inter-project Billing setup

We have implemented inter-project billing between 2 operating units. A project was defined without the "Bill another project" box checked, and the associated project was not defined. The project has had transactions hit and be costed, and had revenue and an invoice. That box is now not updatable. There has only been one invoice processed on the project, so we tried to do a credit memo, and pushed it to AR and tied it back to PA. That field on the form is still not updatable.

Is there a way to fix this, and what is the point where that field is no longer updatable? When funding attaches? When transactions are costed? Something else? Or do I need to request a datafix from Oracle?