There is some incorrect info in this thread. Here is the deal:

1) Award bucket is "N" for coach and "R" for first

2) If you try to change an award ticket outside of the 3 hour window, the agent will need to look for "N" or "R" inventory on your desired flight (and the same tier for N ... there are "low", "medium" and "high" in N).

3) If you try to change an domestic award ticket within the 3 hour window this is SDC. The agent (or kiosk) will only need to find one Y seat for sale on the flight. Net, if Delta is selling one full Y coach seat on the desired flight (Y1), then you can SDC to this flight. The computers treat an award ticket exactly like a revenue ticket for SDC.

4) You can't do SDC on any reservation that has an international segment (even changing just the domestic segment). There have been some reports of agent doing a manual override, but the system by default does not allow it

5) If you are booked on a first class award ticket (R) you can do SDC to another flight (within 3 hours) that is selling F seats (even if R class is not available). Remember if you are upgraded (V or G) you loose your upgrade if you SDC to a different flight

Hopes this helps.

Happydad