If you are in a federal union (like AFGE) your employer (.gov) is attempting to change your working conditions without due process and without a renegotiation of the contract.
Talk to your union steward asap.
If they give you a hard time talk to your local president.
If that does not work your next conversation should be with a labor attorney specializing in union employees.
You have many options.
Just think of fighting this as your full time job and you will win.
Non union