Question: Can I re-assign an assignment which was already submitted by a student?
Yes, you can. If the activity was previously assigned, an i icon will be displayed next to the activity in the Course tab. You can assign it again, when the due date for the assignment has passed, in other case, you need to delete it or change the due date.
To see how to re-assign activities from the Gradebook, see support article: Assigning activities from the Gradebook.