The general principle is that both parties to a contract have to agree to a change.  The best evidence will be a written agreement.  Often changes will be made such as the introduction of a pay rise and there will be no written agreement to this but the parties will be seen to have accepted it by their actions i.e. the Employer paying the new sum and the Employee accepting it.  If there are detrimental changes to the Contract of Employment an Employer should seek the Employee’s consent.  The fact that the Employee has not protested will not in all cases mean that the change is accepted, especially where it is a change having little or immediate impact on the Employee.  In some circumstances such as where there is a transfer of a business there are special restrictions on the ability of the Employer to change the Contract of Employment of the Employee if the reason is related to the transfer.