Not allowed to migrate contract!!

I have been a client of Telkom mobile for the past 23 months. When I took out my contract, the cost was R499 for 10GB data over 36 months. They now advertise 50GB data for R699 over 24 months. After requesting to migrate my contract to the higher price and data contract, I was told that I cannot do it, since this is a 24 month contract, and I am on a 36 month contract. If however my original contract was over 24 months, or if they offered this contract over 36 months, they could migrate me without any issues. How can this be a valid response, and what happened to the CPA (Consumer protection act). Only way they suggested I move is to cancel my existing contract, as a cost of close to R20000, or take out another contract! I do not believe that this complies to the CPA, as I am being forced to stick with a contract, even though I want to move to a MORE EXPENSIVE contract, but I'm refused to because of a contract term issue on their side! I am now forced to buy additional data at a cost of R499/10GB if my data runs out, simply because of their non CPA compliant rules!