Let's imagine a scenario where you setup gateway routing for a PayPal gateway --> Subscribed several customers via that gateway --> Delete the gateway --> Notice that recurring payments are no longer processing and are still being sent to the old gateway. How can you fix this?
PayPal billing agreements are only able to be processed through the PayPal account through which they were originally created. In this scenario, if the billing agreement the merchant is using now was originally created through the deleted PayPal instance, it will continue to route through that original gateway. If you have updated your PayPal gateway account with a new account, you'll need to have the customer update their PayPal billing information on record so that a new billing agreement can be generated through the new PayPal gateway.