Emploer is changing commission remuneration from T4 with employer contributions to OAS, etc to full commission paid to employee having, havind to deduct and remit CRA contributions - after 15 years with company

It sounds like your employer is changing you from an employee to contractor. You have no obligation to accept such a significant and fundamental change to your employment. I recommend you contact our firm to discuss the matter in more detail and see how we can assist you handle this situation.