Question 36:

We raised Credit Note to our customer as a Rate Difference against our Sales invoice.
We know that against which GST Service code we issue this credit note.

Answer 

Question of change in HSN/SAC does not arise and this will remain same as in original invoice for supply for which credit note u/s 34 of CGST act is being issued (for rate differences). (reply dt. 05/08/2018)