I understand why you believe it is not fair. If there is no reason to believe the employer engaged in unlawful discrimination or retaliation though, such that the employer singled out employees based on a protected status to receive larger pay reductions, there would be no basis for seeking legal action against the employer typically.
As employees at will, the employer is normally permitted to change the terms of employment at any time, including modifying employee wage rates as long as they are reduced below minimum wage/overtime laws.
If the wages were reduced below those requirements, then the labor board could take action and a claim should be filed with that agency.
If unlawful discrimination was involved, then a claim should be filed with the state fair employment and housing commission by the affected employees.
Finally, for those employees whose pay was reduced by 20% or more, they would typically refuse this new wage rate offer, quit their employment, and collect unemployment benefits.
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