Is it legal for employer to make impossible for employee to find employment in industry after 19 yr?

I started with a company in 1996 without a non compete. I worked in the same industry for 6 years previously. In 2007 I was forced to sign or lose my job. At that time I was an inside sales manager. The company has changed my duties several times since then. The latest of which in 2009 to an outside sales position. This again was not my choice. My non compete states "The employer has a right to enforce its rights under this Pragraph (f) Any future change in the Employee's position, title, compensation, and/or duties will not release the Employee from his obligations under this Paragraph and the preceding Paragraph." I have 19 years experience in this industry. How can they prevent me from working internally and externally in my field of expertise?