Drupal 6 to 7 Upgrade Path

Related: #635104: default.settings.php should document how to override strings with context

/**
 * Update to Drupal 7. This will add the context support to custom strings.
 */
function stringoverrides_update_7000() {
  $ret = array();

  // Retrieve all existing overrides.
  $or = db_or()
    ->condition('name', 'locale_custom_strings_%', 'LIKE')
    ->condition('name', 'locale_custom_disabled_strings_%', 'LIKE');
  $result = db_select('variable', 'v')
    ->fields('v')
    ->condition($or)
    ->execute();

  // Push the overrides one level deeper for context support.
  foreach ($result as $variable) {
    // Retrieve the value using variable_get so that it's unserialized.
    $overrides = variable_get($variable->name, array());
    // Save the value back into the variables table with the context support.
    variable_set($variable->name, array('' => $overrides));
  }
  return $ret;
}

Committed.