Here, from UCAN (Utility Consumers' Action Network):
They give examples of fee changes, but this may qualify, too, if it severely restricts your rights. I do not know, IANAL.
http://www.ucan.org/telecommunications/wireless/how_to_cancel_get_out_of_your_cell_phone_contract
Also, it does not look like it is LAW that allows this, it is your phone contract. So read your contract, this change MAY be a free out.
Contracts vary, but most changes to your contract can provide grounds to get out. Almost any new fee or new charge is your opportunity to break the contract. Why? Because they can't change any part of the contract without your permission. But they do it all of the time because they know that you DON'T know that you can use that change as a basis for canceling your contract.
They give examples of fee changes, but this may qualify, too, if it severely restricts your rights. I do not know, IANAL.
http://www.ucan.org/telecommunications/wireless/how_to_cancel_get_out_of_your_cell_phone_contract
Also, it does not look like it is LAW that allows this, it is your phone contract. So read your contract, this change MAY be a free out.