Gst migration

Yes, As per paragraph 3 of Schedule V of Model GST Law, every person who is registered under any earlier law before the appointed day, shall have to register themselves under GST before the appointed day. Accordingly your client has to migrate to GST because he is registered under Service Tax. After migration to GST, you can surrender the GST registration because your client turnover will not cross the threshold limit of 20 lacs.

Yes every person who has taken registration needs to get registered under GST under the specified time limit.