Composition scheme from vat

Hello,

1.) If a person transfers from VAT under GST composition scheme and taken all closing stock credit in VAT and filed return of VAT, how he will proceed under GST composition scheme? Whether he has to pay tax revising VAT return and then transfer stock or whether he can transfer stock now and then reverse credit and pay tax in GST? 

2.) If pay in GST when he should pay that tax? before 20th August or when filling GST Trans return before 30th september 2017?

3.) If suppose that person holding some stock of interstate purchases on 30th June 2017 can he opt for composition scheme?

Thank You

Regards