Can a broken promise negate a noncompete?

My employer reneged on his promise to offer me a partnership after three years as an associate. Instead, he offered me a productivity bonus, and I accepted it. For the next four years I stayed with the practice, and now I want to find a new job. Am I still bound by the original noncompete agreement I signed when I joined the practice? It says I can't work in the area for two years after I leave.