Theanswer to this is 'it depends'.
It depends on a lot of things, the reasons for the change, are the employers expectations realistic, is the employee reasonably able to do the changed job, is the JD contractual, what else does the contract say, what are the alternatives, etc etc.
For example, if you work in a widget factory and your job is to press button A 50 times a day, and they want you to change to pressing button B 50 times a day, then all else being equal, that should be fine. If pressing button B 50 times means climbing a ladder to do it each time, and you are not capable of doing that due to a protected characteristic such as disability, then it's not going to be fine, and could be disability discrimination to try and impose it on you. But that in itself depends on a lot of factors.
If you work in a widget factory, and your job is to press button A 50 times a day, and they want you to change to something fundamentally different, say, building the circuit boards that go into the widgets. But with no training, no experience of building circuit boards, and without giving you any tools. In that case, then no, that would not be reasonable. If they were offering the training, practice and tools, and giving you reasonable time to get up to speed, and offering topay you the same as the other circuit board makers, assuming they get more money, then it might be reasonable. But there's a lot of depending factors in there. If the reason for the change is to keep you in a job because Button pushers are being made redundant, then that might make a difference too.
In terms of what you do about it, if you're not happy then you have the same options you always have at work when you are not happy. The simplest is to have a conversation with your manager, or the person making the decision. Try and reach a compromise, make it clear what you are and are not capable or willing to do, while being prepared to meet in the middle somewhere. Most employment contractshave a clause which suggests that employees will need to do any other tasks as deemed necessary by the company, which is a big catch all for this kind of thing, so it's unlikely you will be able to just refuse the changes. A more formal approach is to invoke employment policies. If they have made a fundamental change to your job role, and you can't do your job as a result, then you can consider a formal grievance.
The nuclear option is to leave. Which, if you can't reach agreement, and you can't find grounds for a grievance in your employment policies, will be the last resort.