Can't Change Assignment Type to Group Assignment

I created an assignment in which I inadvertently failed to post date the start date until after I created groups for the class. A student submitted his work, which I did not grade, and the Assignment Type setting locked to Individual Assignment. I deleted the student's submission and changed the start date to a later time but I still cannot change the assignment type to group. Short of creating a new assignment, how can I unlock the Assignment Type setting so I can change it to group?

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