a term in an employment contract giving the employer the right to make changes to the geographical area to which a commission earning employee is assigned is subject to an implied duty on the employer to take account of the financial impact of making such a change. Failure to do so can amount to a fundamental breach of contract entitling the employee to resign and claim constructive dismissal.

For main relevant notes see Changes to terms of employment and/or Implied terms in employment contracts/duties of employer