Whether all assesses/dealers who are already registered under existing central excise/service tax/ vat laws will have to obtain fresh registration

No. Every person registered under an earlier law and having a Permanent Account Number issued under the Income Tax Act, 1961 shall be granted registration on a provisional basis and a certificate of registration incorporating the Goods and Services Tax Identification Number (GSTIN) therein, shall be made available on the Common Portal. Further, the proper officer will issue a final registration certificate after calling for information and documents.

(ICAI FAQ PUBLICATIONS 06-09-2017 REGISTRATION: FAQ NO. 34)