Gst on chartered accountants

CA firm Initially registered under Service tax and migrated into GST. It has gross turnover less than 20 lakhs and has Provisional ID GSTIN. My questions are- 1. Whether Firm is required to maintain GSTIN . 2. If not required how Provisional GSTIN to be surrender. 3. Whether having GSTIN, it may possible to invoice without charging GST to its Client. 4. Running with GSTIN Return to be filled as NIL as before ST. 5. If GST applicable what will be ASC code for different services in the profession and applicable rate. ABOVE five questions are based on the basis THAT FIRM HAS AN EXISTING GSTIN AND GROSS TURNOVER BELOW 20 LAKHS.