Unilateral change of contract terms

I have a 4MB Telkom fibre line. I have recently be informed that they will be automatically upgrading me to 10MB and charging me more for the privilege. I do not want or need the upgrade and certainly do not want to be paying more. On phoning to inquire, I was told that the issue was out of their hands - and my only option was to accept the change or cancel my service.

I'm quite sure I don't have the power to unilaterally change the conditions of a contract, so I'm not sure why Telkom has.

I'm seriously considering cancelling.