Renegotiating FPIF CLINs

Normally, when I think FPIF, I think "contractor overrun" -- but here's a new one (for me): the contractor is actually significantly _under_running because of unexpected production efficiencies realized during the multi-year procurement (for production, not services). The contractor is worried because it believes that the CLINs are now over-priced.  As it delivers, it's billing more than will be the case when the Incentive Fee formula is used to determine the final price. That's a great situation from a cash flow perspective, but not so much from a revenue recognition perspective. The contractor is convinced that, at the final Incentive Fee reckoning, it will have to send its government customer a Very Large check, which it would very much rather not do, even though it means that the government is basically giving it a nice, interest-free, loan at the moment.

Instead, the contractor would like to reprice CLINs to recognize the underrun. However, the contracting officer is reluctant. 

Is there something I should know to understand why repricing CLINs on an FPIF contract -- to recognize an underrun and to prevent what could be perceived as an over-billing -- is not favored?

Thank you.