I may need to cancel a Delta domestic, first class, one-way, nonrefundable ticket for later today (Jan. 18). Please correct me if I'm wrong, but as I understand it I can cancel and will get an e-credit for the value of the ticket. I have a few questions:

1. Right now I don't know where else I'd fly using the ticket funds. But is there any advantage to changing rather than canceling the ticket?
2. Am I correct in assuming that I need to use the funds within a year of purchasing the ticket?
3. If the answer to #2 is correct, does that mean I simply need to book the new flight within a year or that I must also fly within a year?
4. Is canceling complicated by the fact that I've already checked in? If so, what do I need to do in order to cancel?
5. Is canceling complicated by the fact that I purchased the ticket from Delta but used a partner airline's frequent flyer #? (To be clear, I am flying on Delta metal with a Delta flight number, not a code share).

Thanks very much for any help.