Singapore allows same-day ticket changes within 24 hours of the flight's originally scheduled departure time. Same-day flight changes are subject to seat availability and are only possible under the following circumstances:
Change Singapore Airlines Flight says If you don’t change or cancel your ticket before departure, it will have no remaining value later.
If your new flight ticket is cheaper, the difference will be refunded in the form of a credit for future use.

How to Change Singapore Airlines Flight

Flight policy changes allow you to change flight dates, times and destinations.
Flight changes can be made using the 'Manage Booking' option on the Singapore Air website.
If your flight has already departed, you can change the date and time.
If passengers are traveling in a group, a change to one flight reservation will apply to the entire itinerary.
Singapore Air's dedicated staff and representatives are available to assist you with flight changes and other booking-related questions. Passengers can choose to change their flight through online or offline methods. Passengers only pay the fare difference and the corresponding fare. All flight changes must be made at least 4 hours before flight departure. Once you check in with your boarding pass, you cannot change your flight. Flight changes are only permitted after full payment of the reservation with Singapore Airline.