I do them all the time as well, but I was told recently that even if the award was a low (green) reward there has to be an award seat on the plane. It could be high (red) just as long as there is an award seat available.
I have seen flights close to departure that did not show red availability but did have a Y seat for sale. So according to the agent SDC would not have worked.
This is inaccurate; the new flight only has to have the same class of service (Y vs. F/J) available.
Yes, you can most definitely SDC award tickets.