Thank you for the information and your question. Unfortunately, unless you have a term contract that locks in your hours, duties, pay, etc. for a specific time, or you are part of a bargaining agreement that guarantees the rights you are asserting, there is nothing in the law that would support your position. In other words, as an "at will" employee, your employer is always free to set, and change, the terms and conditions of your continued employment as they deem necessary. A fairly one-sided arrangement, but that is the state of non-contractual employment in Colorado and most other states.
So, unless you are being treated less favorably than other employees simply because of your gender, race, national origin, age (over 40), disability, religion, pregnancy or military service, you would have no legal options in this case other than to leave employment or accept their terms if you cannot negotiate anything closer to what you are wanting.
Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the box above, I will receive credit for assisting you today. Thank you