The following response is based solely on the question and background information provided. As we do not have all of the facts particular to your contract, program, and situation, we highly recommend you consult your Contracting Officer and Legal Office for guidance.
You are looking in the FAR to find a citation that prohibits you from using a contract type that is not descriptive of the actual agreement between the parties and the actual compensation arrangement for the work that was done. You will not find such guidance any more than you would find guidance such as "do not enter into illegal contracts." You cannot change the contract type after the fact to one that does not reflect the true compensation arrangement simply to expedite close out.