Originally Posted by OptionsCLE

As others mentioned, your SIN-JFK won't help you if you want to change the SQ portion to SIN-FRA.

Say for the sake of argument you held SIN-FRA-JFK, not the single segment SIN-JFK thru-flight. Any other major airline would allow you to swap out the FRA-JFK and recognize that you already have confirmed space on SIN-FRA. SQ, however, would require that SIN-FRA were available at the time of your request for a change.

Since SQ typically makes only one F seat available per flight, it's essentially impossible for one to make a change which maintains the original date of travel.