If your employment contract includes a unilateral changes clause, your employer may – under circumstances – unilaterally change your employment terms. He may do so if he has a substantial interest in the change and if his interest outweighs your interest.
In case no unilateral changes clause is included in your employment contract, your employer, in principle, may not change your terms of employment without your consent. Nevertheless, the employer may in some cases unilaterally amend the agreement. An employee must behave as a “good employee” according to Dutch law. If your employer makes you a reasonable proposal which you cannot reasonably refuse, being a good employee involves that you must agree to it.