Unfair contract obligations

Evotel has the infrastucture for Fibre in my area. Last year I entered into a 24 month contract for 20/10 for R729 p/m. I understand that because of interest rates and CPI and BLAH, BLAH that my service is going to cost be a tad bit more. (I got the email 2 months ago) Now Vodacom tells me that they no longer support the BANDWITH that I have entered into 24 month contract. Vodacom expects me to pay the diffierence on their decision they made. I contacted Evotel and they sent me a list of SP prices, incl. Vodacom is trying to milk.