Create a sub-task if custom field is changed

Use a ScriptRunner event listener to listen to all issueUpdated events in JIRA

Solution

  1. Use an event listener to listen out for the custom field value to change
  2. If the custom field is updated, then set the event listener to create a sub-task

Step-by-step guide

  1. Choose JIRA Admin --> Script Listeners --> Create a subtask --> Expand Examples
  2. Use the "check for field changes"

changeItems.any {
    it.field == 'Incident Priority' && it.oldstring == "Priority 1" && it.newstring == "Priority 2"
}

The temptation in this instance may be to set up a post function during the create transition (see the related Atlassian Answers) but that won't help for changes after the issue is actually created.