Yes, an HSG-II/HSG-I/LSG/PA official can decline his promotion after completing three years/one tenure or at any date after accepting the promotion. According to FR 15(a)(2), an official may revert from a higher post to a lower post, and their pay will be protected. However, further increments will not be drawn until the official reaches the equivalent pay slab in the lower grade, so there is no question of a downgrade in pay.
For more detailed guidelines, refer to the Dept of Per & Training Office Memorandum No. 16/6/2001-Estt-PA-1 dated 14.02.2006. You can download the application form for reversion Below.
