Well, they can sell their holdings of US bonds on the market. That's a matter of indifference to the US government, because it doesn't affect the repayment schedule at all.

It might affect the price of the next sale of Treasuries, but that's the limit of its impact.

Thaty's about it, and it might not even have too big an impact on the next sale, since those would have different maturity and won't be equivalent.