Upgrade without my consent.

I upgraded my data contract back in November 2023 which we had an agreement that I have to pay R589.01 on a monthly basis for 24months. I recieved my invoice for the month of April and it states that I need to pay R646.13 , when I called to enquire about this, an agent confirms that my 30GB contract was upgraded to 50GB. How is this done without my consent? Who did Vodacom communicate with regarding this? I am not satisfied with the service I am currently getting. Can this be rectified as I am not willing to pay the R646,13. The only amount that I am willing to pay is R589.01 that we agreed on.