A lot will depend on how they go about it IMHO .. when I worked for the Courts I was in a slightly similar position, I was employed as an Usher in the Crown Court, complete with job description. When a Clerk in the Civil court left I was "told" I would now cover that position as well. A polite complaint regarding pay, responsibility and availability, got the same answer "and any other work within the role deemed necessary for the organisation"... so I did both ....
There was then a complaint from a Judge about my "absence" from a case when he "needed" me, so a "disciplinary" was raised against me .... and the fun started
.
I made it ride, right the way to the disciplinary hearing, at which my line manager attempted to make it my fault for not "managing my time" correctly meaning I was "unfit for the role" ......
At this point I produce the TWO job descriptions, my original terms of employment, which stated exactly what time periods I would be in the Crown Court and the JD for the position I was also covering .. which stated exactly what times the Clerk would be available in the Civil Court......
I simply asked the adjudicator just how I was supposed to be in 2 places at once, along with a copy of my letter asking about pay, responsibility and availability. It all went rather quiet.
My line manager was demoted and replaced. I was given a pay rise for increased responsibility, and my new line manager did what SHOULD have been done in the first place .... she rewrote the JDs so that both jobs were merged and could be covered by one person with one JD.
So, in short, if they force you to take on the new role as well, ensure you have a JD that covers your positions in both roles and lays out EXACTLY what you are required to do, and to what standard.
Good luck !!