Join Date: May 2004
Location: SIN (LEJ once a year)
Programs: SQ, LH, BA, IHG Diamond AMB, HH Gold, SLH Indulged, Accor Gold, Hyatt Discoverist
Posts: 8,817
Plus, if OP is a frequent UA flyer, don't assume the same day standby policy that UA (or other US carriers) offer on their own metal, extends to flights on other airlines like SQ.
As Kacee mentioned, the transfer desk or check in in SIN "may" entertain the request, but granting it is unlikely and they may mess up something when they fiddle with your UA ticket (or they may just see 016 ticket stock and step away from it). I'd second the suggestion not to touch it.