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That will depend entirely on the nature of the employment itself. In Texas, they recognize the concept of "at will" employment. That means that the terms of your employment can legally be altered at any time, with or without cause. There doesn't need to be warnings, write ups or anything of that nature. The terms of the employment can just be changed and, in fact, the job itself can be terminated also without cause.
The only way to be outside of this "at will" employment status is to have an employment contract specifically stating that you can only be terminated for cause, or that the terms of your employment cannot be altered without your consent. Then you'd be able to sue for breach of contract for any change of terms without cause.
In the absence of a contract like that though, the changes would be legal unless you can prove that the actual basis for this treatment is your race, religion, gender, age, disability or recent FMLA (medical leave) use.
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