If you do not migrate to GST. Your registration will stand cancelled automatically after few months.
VAT is no more a valid law and taken down so registration under VAT has no legal validity now.
But it is better to migrate to GST and then cancel the provisional registration on GST portal.
Though you will be liable to pay any unpaid liability and keep the books of accounts for 6 years (depends on your state vat law).
VAT department can still open assessment for past years.