To reuse an answer key from a previous assessment...
Go to the assessment that contains the answer key you wish to reuse,
Click the 'Duplicate' button located under the assessment header,
Name the new assessment and select the desired course.
A new assessment will be created in the selected course and the answer key will automatically be copied to the new assessment.
To reuse an answer key from an assessment that is listed as a 'Past Assessment', Select the 'Results' menu dropdown arrow and select 'Duplicate Assessment'. You can then enter the new assessment name and select the new course.