Point of a contract

Vodacom need to look up the word meaning contract! I have numerous amounts of contracts with Vodacom and have been with them for many years, suddenly i get a notice in the post stating that my contracts will be increased, however i have signed a contract, and for a certain value for a certain amount of years, i queried this with the call center, and i was simply told that i can cancel my contracts if i was not happy and that it states in my policies that Vodacom may increase my contracts at any time, i then asked for a copy of where it states this, however they could not produce a signed copy of this. I am in the process of going over to anither network, who offer to pay up your existing contracts and move over, this is not the first or second incident i have have with Vodacom, the service level and attitude is pathetic, to tell a loyal customer with numerous amounts of contracts and who has supported Vodacom for years to simply cancel is not on at all.