wp-utf8mb4-to-utf8general.php

<?php
/*
* Upload it to the same directory as wp-config.php and run it vía web
* 
* Original idea from http://www.sanjaybhowmick.com/how-to-convert-mysql-collation-from-utf8mb4-to-utf8/
*/

include_once 'wp-config.php';

$dbname = DB_NAME;
mysql_connect(DB_HOST, DB_USER, DB_PASSWORD);

mysql_query("ALTER DATABASE `$dbname` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci");

$result = mysql_query("SHOW TABLES FROM `$dbname`");
while($row = mysql_fetch_row($result)) {
  $query = "ALTER TABLE {$dbname}.`{$row[0]}` CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci";
  mysql_query($query);
  $query = "ALTER TABLE {$dbname}.`{$row[0]}` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci";
  mysql_query($query);
}

echo 'All the tables have been converted successfully to utf8_general_ci';