My question has many parts:
1.Can you change a COST type CLIN to a FFP type CLIN in order to CLOSE out the contract quicker? Does DCMA/DCAA have to review it before you close out the contract - just because it was billed as a cost reimbursable type CLIN? Doesn't DCMA/DCAA have to review the costs to ensure a fair and reasonable price? If you can do this, why don't we do it on all cost reimbursable contracts? Is there a FAR reference that says "Thou shall not" or "Go ahead..."
This answer is only based on the information provided in the question and the question background. You stated you came across a modification changing a Cost CLIN to a Firm Fixed Price CLIN so the contract could be closed out quicker. You also stated it was billed as a Cost CLIN, which means it was a Cost CLIN when the work was done. Then after the contract was completed the Cost CLIN became a Firm Fixed Price CLIN through a modification. A modification can’t be used to go back in time to call a cost CLIN a Fixed Price CLIN to speed up the contract closeout process.
Your second question is; Does DCMA/DCAA have to review it before you close out the contract because it was billed as a cost reimbursable type CLIN? Please see Quick closeout procedures at FAR 42.708.
Your third question is; Doesn’t DCMA/DCAA have to review the costs to ensure a fair and reasonable price? The Contracting Officer is responsible for getting a fair and reasonable price. They should not award a contract if the price is not fair and reasonable. If you are asking about an audit please see FAR 4.804-5(a)(12) which states Contract audit is complete.