Bad service and new FUP

I entered a 24month wireless internet contract in February this year with a FUP which stated I get 300Gbytes data per month at full speed and throttled for the rest of the month if I exceed the 300Gbytes. Since 1 October Sonic has changed this to a 7day rolling algorithm which in effect means I get throttled throughout the month. Being throttled was never an issue for me as I wanted 300Gbytes at full speed then Sonic can throttle me as they see fit. My issue is they have changed the conditions of the contract I have signed up with and I want to terminate the contract without penalty, which they are not willing to do. I feel I have been wronged and should not be forced to accept the new FUP policy, and be able to terminate without penalty. Anyone have advice what I should do. On the other side, their speeds are also not very stable and I had slow speeds right from the start. They even came out to change my antenna angle to another tower. They seem like a bunch of *****s as bad as telkom. Stay away from Sonic/ Herotel