I don’t know that they have “changed the terms”, it just that they advertised it as if it was an actual guarantee to not see price increases, when it was only a guarantee that they would pay customers last months bill if they wanted to cancel if they did increase the price of montly service. It could be claimed it was fraud (with a very low likelihood of succeeding at trial), but not false advertising, if the terms were laid out for anyone to read before they signed their contract.