Increase of existing contract fees

I know that everything have their time for increases, and appreciate it the Vodacom for so long had its prices stable, but we received a letter that from the 1st of May there going to be an increase on all Vodacoms' customers packages. As I say, I will give them credit for new lines to give an increase, but it is lawfully not right to increase our standing two year contracts, because we sighed a contract for a specific amount and by increasing it, they are breaking that contract. I therefor feel it is not right to do it with the contracts before there two year periods are not finished. Will someone please help us. Thank you