Your only saving grace would be contractual fine print, but you don't recall having one.
If you're union, there might be a grievance process or a union contract which dictates what they can or can't change during the contract period.
If you're "at will" they can fire you for anything except "protected class" discrimination.
At least that's how it is in my state (MA).
You'd want to check with your state because some states don't allow the agreements and some are very strict about the wording. (Meaning if it's not written well, it might not be valid.)
Stupid and unfair, yes. Illegal or wrongful, probably not.
But that is my humble *personal* (read=not a lawyer) opinion.