Credit note of pre gst regime

When we add details in Credit/Debit Note in GSTR-1, there is option "Pre-Gst Regime", on selecting that option we can add Invoice of Pre Gst Period (i.e before 1st July). I had give service to my client in the month of June and raised Invoice for the same, but my client asked me to issue him a GST invoice for that service in July. So for the invoice I raised in June which had Service tax, I issued a credit note to my client in July and gave him a new GST invoice. Now my question is, can we show that credit note in GSTR 1 and if yes then, while adding details of taxable value, under what rate should I show the service tax amount? As only GST rates are mentioned in that table. I called GST help desk several times for this query, following are two different suggestions I received: 1. The helpline executive asked me to add service tax amount in the 18% column and said that we'll get an option to reverse excess 3% in GSTR-2 or GSTR-3. 2. Other executive asked me to mention the taxable value of service in 0% column, and said that we won't get any credit for the same, and that information added after selecting "Pre-Gst Regime" option is just for information purpose for the government. Both of these suggestions from the helpdesk executives are poles apart from each other. Does any one have any better suggestion? Thanks & Regards Divyanshu Pathak