After registration of a postpaid mobile bill and changing the network, that billing info will be invalid. Your payment will be lapsed after porting the existing network. In that case, you will need to write an application to the nearest branch to disconnect your payment facility from BillPay connected to your old service provider. Besides, you need to register for a payment facility with a new network provider again. This way, you will have to submit a letter to the branch nearby. You can also login to HDFC internet banking and click “Add Biller”.