Currently it is not possible to amend the parent of a sub task via Jira automation or the REST API and therefore the only way to amend this is manually.

It seems as though this is a feature that would be useful and required by more than one customer.

This has previously been raised for Jira Server; this task is the Jira Cloud equivalent. Please see linked task here:

https://jira.atlassian.com/browse/JRASERVER-68763?error=login_required&error_description=Login+required&state=20ade4a3-41d2-4b9f-ac20-a90cd1112aa5

Update 6 April 2022:

Hi folks, we’ve just released an update to the REST API that should allow you to update the parent of a subtask by updating the parent field of the given subtask via the PUT /rest/api/2/issue/{issueIdOrKey} or PUT /rest/api/3/issue/{issueIdOrKey} APIs.

For an example on how to do this, please see change 2 in this document: https://community.developer.atlassian.com/t/deprecation-of-the-epic-link-parent-link-and-other-related-fields-in-rest-apis-and-webhooks/54048

Please note that rules apply to re-parenting subtasks. In Company Managed Projects, subtasks cannot be the child of another subtask, and they must already belong to the same project as their new parent issue. In Team Managed Projects, subtasks must be a child of a base level issue (by default, a story, a bug, or a task), and must also already belong to the project as their new parent issue.