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It would be possible for you to argue that your employer breached its contract with you. You were guaranteed commission and you were guaranteed a certain sales area and now you are not getting either of those.
If this were to go to court the court would balance your employers right to compete fairly with your right to earn a living. If your employer is breaching the contract and preventing you from earning an adequate living would not be fair for that employer to be able to prevent you from going to work for another employer so that you can earn a living.
However, to be prudent, you should have your noncompetition agreement reviewed by a lawyer that you see for a face-to-face consultation. So your next step should be to consult with an employment lawyer face-to-face.
You should also know that you actually may have grounds for suing the employer for wrongful dismissal should you decide to leave the employer. That is because the employer has altered the terms of your employment in such a way that you could likely argue that you have been constructively dismissed. This will be something to discuss with employment lawyer as well.