Once you check in, you've cashed your ticket. However, lots of people have scammed this flexibility and appear to have gotten their money back. Not sure why you'd want to get so far only not to board. (Cases I know of are people buying the cheapest refundable F ticket to use the SQ or CX lounges at their respective hubs, but never intending to fly. Only FTers would be so perverse. Otherwise, why in the world would you want to do such a thing?