I still have my original contract that was signed when my service was installed several years ago. I have not made or signed any new contracts; so it seems reasonable that a contract, (a legally binding agreement between two parties) cannot be changed, unless both parties agree in writing. At least that's what the lawyers have been leading me to believe.
This is one reason that we have judges, courts of law, and juries. To determine whether or not contracts are legal and binding; and whether or not either party has breached the contract.
Anyway. I'm not an attorney, but I bet you can find one to answer any questions you have about this issue; and they might even give you a free consultation, if you ask. G'luck.
Have a great day!