ARE THE Ts AND Cs OF VODACOM CONTRACTS ONLY BINDING ON ITS CUSTOMERS?

Prior to March 2018 the carryover period for airtime on a uChooseFlexi contract was five months.

From March 2018 Vodacom reduced the carryover period to two months.

Since my contract with Vodacom was signed in January 2018, why has my carryover period been reduced to two months?

It would seem that that the Ts and Cs of a contract can be changed at will by Vodacom, but not by the customer.