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

No, GSTN shall migrate all such assessees/dealers to the GSTN network and shall issue a provisional registration certificate with GSTIN number on the appointed day, which after due verification by the departmental officers within six months, will be converted into final registration certificate. For converting the provisional registration to final registration the registrants will be asked to submit all requisite documents and information required for registration in a prescribed period of time. Failure to do so will result in cancellation of the provisional GSTIN number.
The service tax assesses having centralized registration will have to apply afresh in the respective states wherever they have their businesses.