Jira script runner : change task to SubTask and link to parent

JIRA changeToSubTaskAndLink.groovy

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.ComponentManager
import com.atlassian.jira.security.Permissions
import com.atlassian.jira.event.type.EventDispatchOption
import com.atlassian.jira.user.ApplicationUser;
import com.atlassian.jira.issue.MutableIssue;
import com.atlassian.jira.issue.UpdateIssueRequest;

//Examples, 5 is my subtask issue type id. Will be different for others. Can find it in the issue history tab after converting a task to a sub-task
changeToSubTaskAndLink("XXX-8889", "XXX-0000", "5")

//Method to do all the work
def changeToSubTaskAndLink(parentId, childId, subTaskIssueTypeId)
{ 
    //Get the parent Issue 
    def parent = ComponentManager.getInstance().getIssueManager().getIssueObject(parentId) 
    //Get the child Issue 
    def child = ComponentManager.getInstance().getIssueManager().getIssueObject(childId) 
    //Change the child to the subtask type 
    child.setIssueTypeId(subTaskIssueTypeId) 
    //Update the issue
    ComponentAccessor.getIssueManager().updateIssue((ApplicationUser)ComponentManager.getInstance().jiraAuthenticationContext?.user, 
                                                    (MutableIssue)child, 
                                                    UpdateIssueRequest.builder().build())
    //Create the subtask link, if this is not done you'll end up with orphans
    ComponentAccessor.getSubTaskManager().createSubTaskIssueLink(parent, 
                                                                 child, 
                                                                 ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser())
}