******* change of contract

So I bought a laptop on contract two years ago, and was due for an upgrade in September. When I wanted to upgrade two days ago, I was told, no, they have a new system which only allows me to upgrade in a year's time. I did not sign a three year contract, and how can Vodacom unilaterally change the conditions of my contract? I asked the salesman at Deon Wired to allow me to speak to a very rude Morwa Lephala who became very angry because customers are not allowed to speak to them. Why do I not have the right to speak to a manager if I want to?! Surely they cannot just change the conditions of my contract without any communication in this regard, or me signing any documents? I have been trying to get an answer, but I'm not having any joy.