Epic Link field change is not available as a trigger field in cloud
We currently can not trigger a rule when the epic link field changes. This is due to some limitations in the JIRA API.
It may be possible to work around those limitations to provide support fo this.
Server & Cloud now use issue updated with Changelog.Epic Link https://codebarrel.atlassian.net/browse/AUT-500
We should also support parent/sub-task links as well.
Server (Since 8.4.2) and Cloud now emit issue updated with Changelog.Epic Link, which can now be added to a trigger.