Whether this applies to you depends on what sort of contract you've got. VM's communication leaves a lot to be desired, and many customers are under the misapprehension that they've got a fixed price deal (that remains the same for the full contract term), and whilst that is true for a minority, the majority of customers are in fact on fixed discount contracts. That second and much larger group continue to get their agreed discount for the length of their contract, but VM can increase the pre-discount price. They'll still get their £x a month discount, but the price to which that applies can be raised. This may be worth a read as the new T&Cs will make a material change to how VM's price rises affect you in future years.
