You need to upfare to then available full business class fare. You also need minimum of one extra hour as your whole ticket need to be reissued with new fare calculation.@:-)

It is shocking that SQ thinks they can sell business class seats at full fare. I have two segments waitlisted for D class for over one month now.

See also:
http://www.flyertalk.com/forum/singa...i-airport.html
http://www.flyertalk.com/forum/singa...-business.html
http://www.flyertalk.com/forum/singa...k-jfk-fra.html