Unfortunately, that's the downside of being budget, you either lose that bit that you paid or end up paying more than what you thought you could save.
for full fledge, you can make changes by paying a fee.
if not wrong, cx is 150sgd if u wan make changes.
for budget, it is super hard
think have heard before my friend got refund back, refund is just the fare, not the taxes.
so if u book 5sgd fare, and the tax is 100sgd, they only refund you 10sgd (coz two way) ...