Changing Contract Bundles without consent and charging the same price

MTN changed my monthly bundle from 8GB per month to 6GB without my consent and without changing the price. I tried calling them and they did not provide any help. Instead I am forced to buy prepaid bundles for much more than more contract value in order to supplement this mistake. I was considering upgrading with MTN but I think the lack of customer care is enough to make me move to another service provider.