<?php

/**
 * Implements hook_node_update().
 */    
function mymodule_node_update($node) {
  db_update('node_revision')
    ->fields(array('uid' => $node->uid))
    ->condition('nid', $node->nid)
    ->condition('vid', $node->vid)
    ->execute();
}

/**
 * Implements hook_workbench_moderation_transition().
 */
function mymodule_workbench_moderation_transition($node, $previous_state, $new_state) {
  if (isset($node->nid) && isset($node->vid) && isset($node->uid)) {
    mymodule_node_update($node);
  }
}

Source: http://goo.gl/VT0qS9