No the existing taxpayers under taxes which are to be subsumed under GST and whose PAN have been validated from CBDT database will not be required to apply afresh :

Will existing taxpayers under Central Excise or Service Tax or State VAT have to apply for fresh registration under GST?

Ans. No, the existing taxpayers under taxes which are to be subsumed under GST and whose PAN have been validated from CBDT database will not be required to apply afresh. They will be issued provisional GSTIN by GST portal, which will be valid for six months. Such taxpayers will be required to provide relevant data as per GST enrollment form online on GST Portal. On completion of data filing the status of taxpayer will change to Migrated. On appointed day the status of taxpayer will change to Active and he will be able to comply with requirements of GST regime for payment of taxes, filing of returns etc., on GST Portal.

GSTN has issued Provisional IDs and passwords to all such taxpayers and the same has been shared with tax authorities for conveying the same to the taxpayers. Enrolment of existing taxpayers for GST started at GST portal on 8th November 2016 and by end of March 2017 a large number of them have activated the Provisional ID and many have completed the migration process. More details are available at https://www.gst.gov.in/help