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.