Cell 200 Gig contract

I do have a LTE contract with Cell C since a few years back. When I took the contract I was told in store that this contract can be changed or cancelled within the period of one month. The 100Gig day and 100Gig night time do not suit me anymore and went in store to cancell so that I can upgrade to an uncapped contract. Great was my surprise to find out that I cannot do this anymore because Cell C change the conditions of the contract and I have to pay a cancellation fee of R5 000.00 which unnessary to say not able to pay. I do not have any knowledge hereof and never signed another contract for these changes.