Set this in Scriptrunner's Script Listeners, listen to issue update event to automatically re-assign issue to the Component Owner when Component changed.

AssignIssueToComponentOwnerListener.groovy

/**
 * On Issue Update Event
 * If Component is changed and only one component is assigned
 * then re-assigns the issue to the Component Lead
 */


import com.atlassian.crowd.embedded.api.User
import com.atlassian.jira.bc.project.component.ProjectComponent
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.event.type.EventDispatchOption
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.IssueManager
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.util.collect.MapBuilder;
import java.util.List;
import org.ofbiz.core.entity.GenericValue;
import org.ofbiz.core.entity.GenericEntityException;

List<GenericValue> changeItems = null;
def delegator = ComponentAccessor.getOfBizDelegator()
def user = event.getUser();

def changeAssignee(Issue issue, User user) {
    String currentAssignee = issue.getAssigneeId();
    String componentName = null;
    String componentLead = null;
    Collection<ProjectComponent> components = issue.getComponentObjects();

    for (Iterator<ProjectComponent> iterator = components.iterator(); iterator.hasNext();){
        ProjectComponent component = (ProjectComponent) iterator.next();
        componentName = component.getName();
        componentLead = component.getLead();
        log.warn "component name: " << componentName;
        log.warn "component lead: " << componentLead;

    }
    if (currentAssignee != componentLead && components.size() == 1){

        MutableIssue mutableIssue = issue
        IssueManager issueManager = ComponentAccessor.getIssueManager()

        mutableIssue.setAssigneeId(componentLead)
        issueManager.updateIssue(user, mutableIssue, EventDispatchOption.DO_NOT_DISPATCH, false)

    }
}

log.warn "\n\nEvent: ${event.getEventTypeId()} fired for ${event.issue} and caught by Component Update Check Listener\n\n"

try {
    GenericValue changeLog = event.getChangeLog();

    if(changeLog != null){
        log.warn("ChangeLog: " + changeLog)
        changeItems = delegator.findByAnd("ChangeItem", MapBuilder.build("group", changeLog.getLong("id")));
    }

} catch (GenericEntityException e){
    log.warn "\n\nException: " << e.getMessage()
}

log.warn "number of changes" << changeItems.size()
def componentChanged = false;
for (Iterator<GenericValue> iterator = changeItems.iterator(); iterator.hasNext();){
    GenericValue changetemp = (GenericValue) iterator.next();
    String field = changetemp.getString("field");
    if (field == "Component") {
        String oldstring = changetemp.getString("oldstring");
        String newstring = changetemp.getString("newstring");
        StringBuilder fullstring = new StringBuilder();
        fullstring.append("Issue ");
        fullstring.append(issue.getKey());
        fullstring.append(" field ");
        fullstring.append(field);
        fullstring.append(" has been updated from ");
        fullstring.append(oldstring);
        fullstring.append(" to ");
        fullstring.append(newstring);
        log.warn "changes: " << fullstring.toString();

        componentChanged = true;
    }

    if (field ==~ /[aA]ssignee/ ) {
        componentChanged = false;
        break
    }
}

if (componentChanged) {
    changeAssignee(issue, user);
}