Increase in contract fee

I have three contracts with Vodacom and received messages informing me that they have transferred me to different contract plans and that I will have to pay more from next month onwards. I signed contracts for specific plans. Vodacom would not allow me to suddenly pay less, how can they suddenly expect of me to pay more? When I went to Vodacom Three Rivers to complain, they told me I either have to pay or cancel my contracts and pay the cancellation fees. Why did I have to sign an agreement if it can be broken so easily from one side?