Migration of existing taxpayers: provisional GST registration issued subject to prescribed conditions, convertible into a final certificate upon compliance. Section 139 provides that existing taxpayers with a valid Permanent Account Number shall receive a provisional certificate of registration from the appointed day, subject to prescribed conditions, form and manner; non-compliance may lead to cancellation. A final certificate is to be issued in the prescribed form and subject to conditions. A provisional certificate is deemed not issued if cancelled pursuant to the registrant's application asserting non-liability to registration.