Vodacom’s contract ****

I had a top-up contract worth about R95/month. Vodacom said I can upgrade in Dec 2019, which I did. I started with a new company and needed a bigger contract. So I decided on a Red Select 24 month contract, which started in Jan 2020. The 24 month period is over and I need to cancel this contract. Now Vodacom says I cannot cancel as the previous contract still had 4 months left. I am fine to go back to the previous contract of R95/month, but Vodacom wants me to stay on the expensive Red Select contract. How can they force me to pay 28 month for a contract that was signed for 24 months. Be aware of this **** to upgrade customers 4-5 months before their contracts expire and then force them to pay 4-5 months longer on the more expensive contract you upgraded to. I am sure Vodacom is in breach of contract law here and I hope the authorities will stop this ******* practice