Changes in Contract prices

WE have 6 contracts with mtn, and I am concerned that now that it is time to renew one of these contracts as to how I will be charged in the future. We have always had a 2 year contract that you knew what you were paying for the duration and obviously budgeted accordingly. I want to know what stops you as a company from changing my contract any number of times in the foreseeable future and what recourse I as the client have? I posed this question to the call centre today, and was informed that you have the right to change the contract any time you choose. I am only able to \buy"myself out of a contract by paying for the full two years. This doesn't seem to offer me"