To better understand the answer to that question, it’s important to first know that U.S. courts consider employment to be “at will”.
Under the doctrine that upholds at-will employment in the U.S., employment is presumed to be voluntary and indefinite for both employers and employees in the absence of contracts that specify otherwise.
Consequently, unless you work under a collective bargaining agreement or another employment contract that specifies otherwise, generally, your employer may “legally” change your job description (job duties, title or position) at anytime, just as you may quit your job at anytime if you don’t like the change.
Your employer may not, however, violate a law when changing your job description, job duties, pay, or other terms or conditions of your employment. A few laws and a legal concept might provide relief for you and other employees in this situation.
For example, if an employer intentionally changes the job description of a disabled employee so that the employee can no longer perform her essential job duties because of her disability, then the employee might have a case for employment discrimination under the Americans with Disabilities Act (ADA). The ADA requires employers to reasonably accommodate the disabilities of covered employees, unless it causes undue hardship.
The same goes if an employer adversely changes an employee’s job description solely on the basis of any other protected characteristic, such as age, race or gender: The employee might have a case for employment discrimination under the relevant discrimination law.
Under an employment, labor or whistleblower law, an employer might not get away with adversely changing the job description of an employee as retaliatory “punishment” because the employee reasonably exercised his or her rights under the law. Some of the laws have full retaliation protection covering any aspect of employment, but others have only limited protection, such as for wrongful termination alone.
If an employee quits shortly after his employer so-intolerably changed his job duties that it would have compelled any reasonable employee to soon resign, then the employee might have a case for constructive discharge, a form of wrongful termination.
Back to contracts: If your employer has adversely changed your job description contrarily to the terms or conditions specified in the employment contract under which you work, then you might have a case for breach of contract.
Consult a lawyer to discover whether or not you might have a legal case if your employer has adversely changed your job description, job duties, pay, or any of the other terms or conditions of your employment. If you’re working under a collective bargaining agreement, then it might be a good idea to also consult your union representative.