MTN changing terms of contract mid contract... Unacceptable

I have been with MTN for almost 20 years and have never had a problem until now. However, I upgraded my contract and upon upgrading my contract, was assured that I would be able to (and I double and triple checked) add voice and sms bundles onto my contract (in fact, they added a recurring bundle on in store when doing my upgrade in November). Now, some 6 months later, they tell me they have changed the contracts and you are no longer allowed to add voice and smses onto the account.

Surely when you sign up for a contract and there is an agreement when signing up for the contract, it cannot be changed mid contract??? This is truly unacceptable and it will be the last contract I take out with MTN. 20 years loyalty, and this is how one is treated... MTN does not care for it's customers, just out for a quick buck.