Using DL miles, it should not have cost you any additional miles to re-issue your award ticket to effect a change from Y on SIN-DPS to C on SIN-DPS. An award in F or C on SQ using DL miles includes the same class of travel to the ultimate destination in SE Asia, whether SIN or beyond. That is, however, subject to availability, which has been a real PITA to get beyond SIN, especially to BKK.