Ask for a salary re-assessment first. Your employer should be using one of the standard methods of working out how much a job should be paid, and if they are adding further levels of responsibility, then it my mean that your post goes up a grade or two. At least this way, if they impose new duties on you, you'll be rewarded for them. But they also might realise that they don't want to pay you more, and so scrap the idea.
I would ask for this and tell them that you cannot accept the new role until it has been done.
I would ask for this and tell them that you cannot accept the new role until it has been done.