Expired data contract upgraded without consent

My 2Gig data contract expired about a year ago and I have been on a month to month contract ever since. This contract has been upgraded without my consent and I need that reversed. Early this month (4 or 5 Jan) I went to Killarney mall to have my account unlocked as my data had been used up (a problem I have had since last year). During the process of unlocking it was suggested that I change my contract from open to prepaid to avoid the problem recurring. I was happy with this and was given an unfilled form to sign which I foolishly did without reading. Lo and behold when I got home I got an sms from Vodacom congratulating me for upgrading my contract. When I went back to Killarney a couple of weeks later, the consultant promised to call after talking to the person who helped me. I'm still waiting for the call. My question is why would I upgrade a contract that I been running for close to a year at the same cost? I only needed to stop my data disappearing.