Misinformation on an early upgrade

I did an early upgrade with MTN. I was told that since my current contract expires this year in December, I will only start paying for the new contract in December 2023. Meaning I will be paying the same amount of the old contract but I am allowed to get the new device now. The agent said it to me repeatedly that I will not be charged for the new contract until December 2023. But to my horror and shock, the new invoice shows that I am now being charged for the old contract and the new contract raising what I was paying from R959 to R1300, which is not what we agreed on. When I called the call centre today, the agent told me that I was mislead, by her own colleague. This is not the first time MTN played games with me during an upgrade. After being with MTN for more than 10 years with 3 lines of contracts, I have a good mind to move my business elsewhere. I am so disappointed and frustrated with this company.