Gst 20l turnover

As per 13 oct 2017 notification , for purpose of GST registration inter state service provider 20L limit is to be taken on all India basis.

My query is can a service provider havinf turnover below 20L can opt out of gst?

if yes surrender facility is not availbale and gst return upto october is not filed ??

What are consequenses if we continue gst no and not charge gst in our invoice after 13 oct and surrender after filing all gst return upto 13oct??is it possible?