Fed by itself or working with AAA counterparty offers to sell supplemental bond insurance to investors. (AFLAC concept)
Maybe charge a point and insure up to a price of maybe 99, or whatever combo works.
Worst case the current insurers are downgraded to AA, so they should still be able to cover losses, so risk is minimal to the new insurer, and fees will likely be profits.
Only investors who care would buy it.
Bonds would remain AAA rated.
The key is the current insurer’s capital is still in first loss position, and the current insurance is probably still money good, or they’d be talking about a downgrade way below AA.
And not all bond holders care about AAA vs AA. Only those who care buy the supplemental insurance.