My understanding is that each segment credits for the year of its scheduled departure, so that even if the flight departs late, it would still count for 2013. I would expect the same for ORC if you're rebooked in IROPs but YMMV. (Not requesting ORC would probably mean that the rebooking would credit as 2014, assuming that the new flight is scheduled to depart after midnight.) Direct flights would all credit based on the first departure, so that even after an aircraft change, the flights would credit to 2013. However, connecting flights that are scheduled to depart after midnight would count as 2014.

Remember that DL supposedly doesn't allow you to credit different flights of a single itinerary to different FF programs, nor can you change (or delete?) the FF number after departure of the first flight of an itinerary.

Enjoy New Year's Day in Vegas.