If you're not protected by membership in a labor union or by a written employment contract, then you're employment is "at will." That means your employer's free to fire you, demote you, reduce your salary and/or benefits, etc., at any time without notice for any or no reason, as long as they don't illegally discriminate or punish you for using your rights (such as whistle-blowing or union organizing). It also means you're free to quit on the same basis.
So can they change their benefits policy? Yes. Are you free to look for another job today? Yes. Can you force them to reinstate their old policy? No.
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