person is liable to pay tax after migrating to GST

I am a supplier of repair and maintenance service within my state West Bengal. I was registered with earlier law i.e. Service tax and vat. My provisional I'd and password also provided in service tax portal. my annual turn over is less than 20 lakhs. now my question is:-
1. whether I should migrate to GST as per section 22 of the act,
2. If I migrate, should I pay GST immediately after getting the provisional registration certificate or I should wait till 20 lakhs.
please help.
thanks.