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I am sorry to hear about this situation.
Unless the original contract that you signed allow them to "change the terms," then they cannot do so. The employer is bound by the contract and cannot simply change it at will (again, unless the original contract allows them to do so).
Assuming it does not, then one does indeed have a case for breach of contract. "It has long been the law in Colorado that a party attempting to recover on a claim for breach of contract must prove the following elements: (1) the existence of a contract, … (2) [substantial] performance by the plaintiff or some justification for nonperformance, … (3) failure to perform the contract by the defendant, and (4) resulting damages to the plaintiff." Western Distributing Co. v. Diodosio, 841 P.2d 1053, 1058 (Colo. 1992).
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