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

No, the existing taxpayers 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 for providing
relevant data as per GST registration form. On completion
of data filing provisional registration will get converted
into regular Registration. Further notification giving
timelines will be issued by respective tax authorities.