Jira is flexible, yet complex tool in some cases 🙂

1. In order to make the fields (inline as well) editable, when the ticket is closed, go to Workflow -> Select ‘Closed’ status -> Click Properties
2. There would be flags with bool values: we need jira.issue.editable
3. 
4. In case you see jira.issue.editable is false – either change that to true or delete the property key with prev. value.
5. Don’t forget to publish the updated workflow.