Accepting a discount now means I can't upgrade!

I was informed that I have the option of receiving a monthly discount, before accepting this offer I confirmed whether I would still be able to upgrade, the sales person assured me that I would. To my surprise, I was informed today that accepting the discount meant that I am not able to upgrade for the next two years. I was also informed that should I wish to cancel my contract, I would be liable to pay a penalty. I have tried to contact various MTN call centres none of which were reachable. Could someone please get in touch with me and pull the call where I was offered the discount and promised that I would still be able to upgrade. I don't understand why MTN would not want more money?