I had SQ do this for me in the lounge just before boarding was called - move me from C to F on a UA F award ticket where one leg was booked in C due to lack of availability at time of booking and up to day of travel (as far as ANA showed). However, the SQ agent called UA and after much discussion got UA to re-issue the ticket - so SQ was most willing but he really needed to push UA to make the last minute change. All in all I would not count on it.