The short answer is no, a company cannot change the terms of the employment contract by paying you less than what was formally agreed upon.
The employer should consult with the employee and enter into negotiations to change the terms of the employment contract. Although it could depend on the type of contract that exists.
I have been at the same employ for 16 years and have taken on much more duties than stipulated on my original contract. I told my employer I would like to hand all extra work over to relevant staff as I am not being compensated enough. In turn, he has told me that he would like to hand over all my previous years increases to the relevant staff. I took it as, do all the work or your salary would decrease?