Mtn charging prorata amounts in middle of a contract.

I have a contract with MTN. I had a call from one of their call centres telling me that they were discontinuing the package i was on. They told me that i will be moved to another package at no extra cost. I was not due for an upgrade. Now when they send the invoice i am billed twice for the new package which is more expensive than the previous one, including a pro rata. I did not upgrade, i do not understand the prorata amount. I called one of their so called service champions and she had no clue what happened. It’s very frustrating. This is not the first time Mtn has done this. A few years back they charged over R1000 when i terminated a contract. I never got a satisfactory explanation. Mtn is taking advantage of its loyal customers. It’s sad really