Is it legal for my manager to change my job description?
Generally, your employment is "at-will". This means you can leave when you choose--but also that your employer can change the terms of your employment (including your wages, hours and duties) at any time, for any reason.
However, there are a couple of exceptions, and those arise if the changes are being made for a reason that's discriminatory or in violation of a written contract of employment.
Even if you can't challenge the change in duties, it could provide a ripe opportunity to try to negotiate a pay raise (especially if you'll have to take on a substantially greater workload).
Speak to an employment lawyer about your specific situation so that you can get the guidance you need to determine your best course of action.
Nina L. Kaufman, Esq. is an award-winning New York City attorney, edutainer and author. Under her Ask The Business Lawyer brand, she reaches thousands of entrepreneurs and small business owners with her legal services, professional speaking, information products, and LexAppeal weekly ezine. She also writes the Making It Legal blog.