I realize this has been discussed to an extent before, but I believe this should still become a feature. There are certain moments when it is convenient for the teacher to be able to do this. For instance, I published an assignment before realizing I had not set it as a group assignment. A student submitted another class's assignment to my assignment before I attempted to make the change, and now I can no longer fix it. I cannot unpublish the assignment either, so making a duplicate assignment will result in mass confusion from the students and an explosion of notifications and inbox messages for me.