Generally speaking, in order to make changes to an individual employment agreement, both parties to the agreement (the employee and the employer) need to consent to any such changes.
Therefore, if an employer wishes to change an aspect of an employee’s employment agreement, for example, their hours of work, they must have a genuine reason, must follow a fair and reasonable process, and get the employee’s agreement to do this. But sometimes employment agreements change by mutual agreement between an employer and employee – for instance your full-time chef has now gone back to study and as a result (at their request) you have agreed to a change of their work role and hours.
If by mutual agreement the terms of your employee’s employment have changed, their employment agreement should be amended. This will not usually require you to issue a whole new employment agreement, rather a “variation to the employment agreement” can be provided and signed by both parties, amending the parts of your employee’s employment terms that have changed. The Restaurant Association has a variation to employment template that we can provide you to use in this situation.