The migration from VAT to GST has been initiated by the State VAT departments.
As a first step Provisional Registration number under GST and password has been issued by the respective State VAT Department to the Assessee, which has to be used for the Migration process.
A step by step procedure for migration is explained in the Video Linked below for easy understanding of the Migration Procedure:-
http://tutorial.gst.gov.in/video/module/index.html