Create on Transition for Jira, version 7.5, allows you to copy all the issues from the original epic to the epic being created. As a part of this feature, a new toggle Copy issues in epic is added in the Detailed Fields tab. In one go, you can now create an epic and copy all the issues in the epic using the Create Issue post function.
This toggle is applicable only when:
the issue being transitioned is an epic AND
the selected Issue type in the Detailed Fields tab isEpicor Original issue type (where the original issue type is an epic)