not able to change contribution percentage on contract project while adding new customer

We have a Contract Project and have billed close to 40% of the Contract amount. At this point we have been told by the customer that we need to back out the entire billing and should bill the entire contract amount to another Legal Entity that the customer has created (in other words entirely new customer).
We have credited all 5 Invoices interfaced to AR thus far and backed out the entire funding from the existing Agreement. At this point when we tried to add the new Customer to this Project we are not able to change the contribution of the existing customer (to 0%) and the new customer's to 100%.