How can a contract be varied?

A contract can only be altered in the following ways: By agreement between the parties. Through a collective agreement where there is a provision in the employee's contract, for this to be incorporated, (a reference in a collective agreement may not be enough)

Through an existing term which allows for a variation in working conditions, e.g. flexibility by the employer giving proper notice to end the existing contract and offering a new one incorporating the new terms. The termination will be a dismissal, which could give rise to an unfair dismissal claim if the employee does not accept the new terms and leaves.