Uncalled for increase of fibre after only a month!

I signed up with RSAWeb on the 15th June for R575/month, the contract states that if I have to cancel within 12 months I pay installation costs etc - 28 July I receive an email stating that I'm being billed R675 as of 1 September whether I like it or not, simply because they are upgrading me to 15/15 - I NEVER SIGNED UP FOR THAT! Why must I pay an extra R100 for something I don't want?? This is not what I signed up for - this is disgusting behaviour - as if the SA govt doesn't ***** from us now RSAWeb intends to rip me off too now! Not happy!!