subject to termination?

i need advise please.. i am working as a sales staff and every year I am a replacement to a colleague of mine when he is on vacation. His work is secretary/office staff.

Now, I heard that my colleague will be quitting and I am 100% sure I will be replacing him as no one else in our office is willing to take his job. (that is why iv been the replacement all the time). It is a very stressful job honestly. 

My question is, I will take the job and ask for a better compensation, but if they will not give me and I refuse the job and stick as a sales staff, will that be insubordination? Isnt this a change of job description and since it is a harder and tideous job, I deserve to be properly compensated.. can I bring this to the ministry of labor if ever they change my work and not compensate me accordingly? what are my options? please advise..thank you