Your employer does have the ability to make some changes to your role, but if you can compare your new role to other jobs that make more money and your pay didn’t change, you can claim that you quit because your new situation was one that was unreasonable. If you were hired to do a particular job and your employer changed the position, you can explain that you were very satisfied with your role. Recently your employer decided to change your role to benefit the company and didn’t consider the fact that you were hired in for a particular job.
When you confronted them with the issues, they were ignored and you felt the only thing you could do was to quit. At this point, your employer knew that this change in your role stressed you and that despite your attempts to bring it to their attention, nothing changed. Always explain that you enjoyed the job and were looking forward to a long career there and that you are very disappointed that your job changed.