Thank you for trusting your question to JA today. I am a licensed attorney with over a decade of law practice and over 20 years of experience in the legal field. I’m happy to be of assistance.
Unfortunately, the answer here is not going to be satisfying, because it is one of those odd things in employment law. The employer can do this because there is no law saying that they can't. That's really what it comes down to. The fact that they have not do so before does not mean that they can't make changes. The fact that it is not in their policy also does not mean that they can't do it now. They can change policy through re-writing it or simply through practice.
They can apply this just to nurses, because that is not an illegal form of discrimination. Singling people out based on race, religion, gender, age, disability or FMLA use is illegal, but it is not illegal to single out a specific class of employees.
I wish I could tell you differently here, but the employer can make this change. They have given notice prior to doing so and, on that basis and on the basis of the concept of "at will" employment, the employer can do this.
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