I'd like a kid to go back to an assignment she turned in and revise/edit. At this moment, the only option you have is to clone an assignment, and create a specific one for her. Google Classroom has the option of "returning" items, and giving students the chance to fix their mistakes without having to start all over. Then I can also keep this student's current version with others, instead of making a whole new assignment just for her, or making her redo the entire thing by "unassigning" it.