Reporting back in case it's helpful to other folks.

Called EXP desk twice. Both representatives confirmed:

- I can cancel the flight now and call in later (one rep said as long as within 6 months) to change to a new date. So I don't have to request the change now if I'm not certain.
- I'll have to cancel before the original departure time. Otherwise it'll be no-show and can't be changed later.
- If original fare class is available, only change fee is applied.
- SWU can be used for new flight.