Rookie questions about flight change. Appreciate your patience and insight!

I purchased a non-refundable International round-trip ticket originating from US. I've taken the outbound flights. I'm booked to fly back to US tomorrow. Now I have to reschedule my flight, but don't know to what date yet.

It's a super low fare (change fee is $300; remaining portions of the fare should be lower than that).

- Can I cancel the flight now, and call in next week or even later to change it to a new date (same origin/destination)?
- I'll pay change fee ($300) plus fare difference, right?
- I have confirmed SWU. Will this SWU be transferred to the new flight (waitlisted if no C space readily available)?

Thank you very much!

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Updated after confirming with EXP desk:

- 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.