Contract changed without clients consent

I had a two year cellphone contract with CellC and when the contract ended, I decided to take the month-to-month sim only contract still with CellC. Last week I went to the branch to enquire about a cellphone that I was interested in. And I was told that I have a 'commitment' amount of R8500 that I need to pay first if I want to terminate the contract. I informed them that it was month-to-month and to my horror I was told that CellC changed all the sim only month-to-month contracts and made them 24 months contracts. Apparently CellC sent smses to clients informing them of this change. I am very disappointed at CellC for changing our contracts without our consent. But I refuse to pay the 'commitment' amount as I didn't sign any 24 months contract. I call CellC customer number and was informed that if I upgrade to a cellphone contract, I dont have to pay the commitment amount. I informed the person I do not feel comfortable entering into another contract with CellC. And this is the same situation with my husband.