Unilateral Variation of Employment Contracts
As with any other contract, an employment contract can only be changed with the agreement of both parties. If a variation is necessary you should ensure that proper procedures are followed.
Rules and Procedures: The terms of the employment contract can only be varied with the consent of both parties. It may therefore be advisable for you to specify in advance that certain arrangements, understandings and documents are not part of the employment contract, thus allowing you to change them unilaterally provided you act reasonably.
In particular, works rules or company rules which you wish to be variable at your discretion should normally be excluded from the employment contract - this also means that you will not have a contractual right to enforce them.
Whilst it is generally a condition of the contract that the employment is subject to disciplinary procedures and grievance procedures, these procedures should be non-contractual so that they may be changed at your discretion.
Entitlements: There may also be other aspects of the deal with your employees which you would wish to be with in your discretion as opposed to being a contractual entitlement, e.g. payment of bonuses and other benefits.
Job Description: You may also wish subsequently to change an employee's job description. If the job description is excluded from an employment contract, the change may be made at your discretion provided the proposed work is still within the contractual obligations and there is no change of status.
Ideally, a job description should define adequately the employee's tasks, but should be broad enough to allow for reasonable flexibility on the employee's part. This is particularly important if you run a smaller business where adaptability is essential.
However, do bear in mind that if the job description is over-wide it may be difficult to make the employee redundant.
Obligations to Consult: Consultation with individual employees should be carried out before imposing a variation to a non-contractual obligation in any event.