To allow issue-editing while the issue is closed:
Go to the project Workflows
screen and download it as and XML file.
Then edit the XML file and in the Closed
step:
<step id="6" name="Closed">
<meta name="jira.status.id">6</meta>
<meta name="jira.issue.editable">false</meta>
<actions>
<common-action id="3" />
</actions>
</step>
change
<meta name="jira.issue.editable">false</meta>
to
<meta name="jira.issue.editable">true</meta>
You can also mark or remove this line, but it's helpful to keep it for future needs.
Lastly, upload the edited workflow back to Jira.