Sir you have already migrated I.e. now you are register under GST. and the exemption of 20 lakh for taking registration therefore there is no qus about 20 lakh exemption in your case because now you are registered under GST. so you have to pay GST from first rupees sale. I think it will clear your doubt and my friends who says yes without understand your query. if I am wrong then pls clear me.
Migrating to GSTN portal does not cliffs your hand to make surrender/cancellation application. Within 6 months from the GST appointed day you can make surrender application on the GSTN portal on the facts that you gross/turnover is below 20 lakh (10 lakh for specified states) and you are not making any inter-state supply. Don\’t charge any GST during this period.