Xfinity - Billing change without customer consent and after promising to change by xfinity associates, failed to fulfill what was committed

I am loyal customer of xfinity for more than 2 years. Suddenly i noticed my bill was doubled for next month. Connected with xfinity support and they provided me a different plan which is competitive and I confirmed on chat moving to the other plan. I was told it would be effective in the system within 2 days. Since it has not reflected even after a week, contacted the support again, different person, same set of problem explanation and he listened as if I am chatting for the first time and mentioned he enrolled again and will be reflected in two days. Again not reflecting, I tried calling but I have been forced only to chat by the system. I tried chatting with different associates 8 times, each lasting an hour and everytime conclusion is new request made, previous request escalated and it would be in system in a day. Lost patience asked for supervisor and supervisor says plan is no longer active and they cannot fulfill what was promised. Whose fault it is ? I have seen many customer service and when something is promised they make every effort to ensure it is ensured for the loyal customer. Here, I see a different attitude. Supervisor gave me a plan costlier which I refused to agree. He threatening me saying I do not have an option. I had very high regards for xfinity but the way they handled their customer service and responses, I have to give them negative marks.