They will typically give you some discount, but generally never as good as new subscribers. It's flawed business logic to say 'why can't they give me the same discount as new people, I've been with them for 2+ years'. The ENTIRE reason they give you a nice discount when you are new is b/c they want to capture you as a customer. They are willing to lose money on you the first year or so, but eventually they have to raise the rates. This is seen everywhere- banks offer sign on bonuses only to new customers, credit cards have mileage bonuses for newbies, etc. Sure charter should keep offering some deals to retain you, but these are never as good as the newbie discounts.
If you really want to push them hard, call and say you want to cancel- they will get you to the cancellation dept. They have the power to offer better deals- explain you are paying too much and mention at&t has a better offer- that is your best shot at a real good deal. But if you do that be prepared to cancel and switch. Look at u-verse and direct tv- do you really want that? What about internet? Do you want to ditch tivo? Also, the rates for u-verse/direct tv that you will be quoted are for 'new customers' which means that 1-2 years from now they will also jack up your rates...