Can an employer unilaterally change the terms of an employee's employment contract?

In Farber v. Royal Trust Co., 1997 CanLII 387 (SCC), [1997] 1 S.C.R. 846 at 858-859, Mr. Justice Gonthier for the court stated: Where an employer decides unilaterally to make substantial changes to the essential terms of an employee's contract of employment and the employee does not agree to the changes and leaves his or her job, the employee has not resigned, but has been dismissed. Since the employer has not formally dismissed the employee, this is referred to as "constructive dismissal". By unilaterally seeking to make substantial changes to the essential terms of the employment contract, the employer is ceasing to meet its obligations and is therefore terminating the contract. The employee can then treat the contract as resiliated for breach and can leave. In such circumstances, the employee is entitled to compensation in lieu of notice and, where appropriate, damages. … To reach the conclusion that an employee has been constructively dismissed, the court must therefore determine whether the unilateral changes imposed by the employer substantially altered the essential terms of the employee's contract of employment. For this purpose, the judge must ask whether, at the time the offer was made, a reasonable person in the same situation as the employee would have felt that the essential terms of the employment contract were being substantially changed. The fact that the employee may have been prepared to accept some of the changes is not conclusive, because there might be other reasons for the employee's willingness to accept less than what he or she was entitled to have.