Well, if your booking was confirmed then it is the issuing airlines responsibility to either rebook you in F or to compensate you for flying in C. Even if it's an award booking you paid for these flights with your miles. And even if your flights hadn't been confirmed you, of course, still wouldn't have to pay for an F flight while travelling in C.
As the majority of my legs are still in F, US refuses to refund any miles, even if they agreed to refund some of the miles, weŽd be talking about 5k-10k miles. (F award is 150k miles, C award is 130k miles)