Currently, if allow resubmissions has been selected and the due date has passed, there is no way to allow resubmissions without first going to the assignment settings and unchecking allow resubmissions (which restores the allow resubmissions button for the teacher).  In my opinion, the allow resubmission button should be there for the teacher regardless of the settings for the student.  This way the teacher can help a student if they would like to.