Yes. That does not mean that terms may be changed unilaterally (your question) but that the bargaining power is such that the employee may be in a weak "take it

Yes. That does not mean that terms may be changed unilaterally (your question) but that the bargaining power is such that the employee may be in a weak "take it or leave it" position anyway. In that case ultimately it is for both sides to decide based on how easy it is to find a new job/replace employee.