Confusion about open/closed principal

Open/closed principle states that classes are closed for modifications but open for extensions. Lets say we want to design a payment system where payment can be processed by multiple processors like following:

    class Payment {
        void pay(paymentMethod) {
            switch (paymentMethod) {
               case 'PayPal':
               break;
               case 'Swift':
               break;
               default:
               break;
            }
        }
    }

    class PayPal {
        void pay() {
               //implement payment1
        }
    }

    class Swift {
        void pay() {
               //implement payment2
        }
    }

Let's say we implement both payment systems the first time. Now if for some reason any payment system implementation process is changed, should not we have to modify the relevant class? For example, if we implement PayPal and after 2-3 years PayPal's working process is changed, does not modifying the PayPal class break the open/closed principle? If it does what's the solution?