Search API Solr - index one node type as another

foo.module

<?php

/**
 * Implements hook_search_api_solr_documents_alter().
 *
 * Legacy pages should simply be indexed as normal pages.
 */
function MYMODULE_search_api_solr_documents_alter(array &$documents, SearchApiIndex $index, array $items) {
  if ($index->item_type == 'node') {
    foreach ($documents as $document) {
      if ($type = $document->getField('ss_type')) {
        if ($type['value'] == 'legacy_page') {
          // Set index value for this field to 'page'.
          $document->setField('ss_type', 'page');
        }
      }
    }
  }
}
?>