If you UPDATE a Post-Acceptance Question after an applicant has completed the previous version of the question AND you want the applicant to update their answer, you will need to ensure the Post-Acceptance End Date for the Opportunity is still open or re-open it. If it is closed, the applicant will not be able to access the question in their applicant portal in order to update their answer. 

If you ADD a Post-Acceptance Question after an applicant has completed the post-acceptance process/been awarded AND you want the applicant to include an answer to the new question, you will need to ensure the Post-Acceptance End Date for the Opportunity is still open or re-open it. If it is closed, the applicant will not be able to access the question in their applicant portal in order to update their answer. 

If it is not necessary for the applicant to update their answer, no further action would be required by the applicant.