Change of contract cost midway

I am at a loss to understand the meaning of a 24 months contract if Vodacom can unilaterally change the subscription half way into the contract. I have a 2 gig data subscription with Vodacom which is ongoing. I have received a text message that my subscription will now beR109 per month as against R99 from May 1st, 2015. I dont understand if its part of the T&C that they can unilaterally effect a fixed contract rate anytime. <br> If we are to believe that this can be changed anytime, when will I get an sms from Vodacom the my 2nd contract of Uchoose 200 is now reduced from R199 I took the contract in December 2014 is now R139 per month for 24 months which is the current subscription.<br> Its will be only fair that both parties to a contract smiles and not VODACOM SMILES AND CLIENTS CRY.