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This is all highly frustrating, I am certain, but unfortunately the general rule in Texas is that employment is "at will" absent an express agreement to the contrary. At will employment can be terminated for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even true.
Since employees can be terminated for unfair reasons, no law regulates writeups, changes in job descriptions, or things along these lines. These are just internal personnel matters unless you can prove that they are motivated by a legally protected trait or activity as defined above.
It certainly does seem that your employer is trying to push you out, but that's not against the law, since they could simply choose to terminate your employment at any time if they wanted to. All you can do is attempt to reason with your employer. If you are let go, you should have no trouble getting approved for unemployment benefits as you search for new work.
I hope that you find this information helpful and am genuinely sorry if it is not what you were hoping to hear. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
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