UA are apparently currently under enhanced security at Changi (due to an 'incident') and security requires a normal boarding pass (not email printed) to get airside. Also, a UA checkin agent at Changi told me that you can't turn up in the morning for a boarding pass for the UA evening flight (same day) even if no bags to check - and the UA checkin agents don't turn up until 3 hrs before the flight... (Not sure if codeshare can bypass the restrictions). Hmmmm Changi is my favorite transit airport .... but this is a bit of a curve-ball.
I haven't bought and cancelled a different airline ticket just to get through security for a while but I guess that is one way round...