While the database upgrade is the most important step when moving from one major version to another, it's worth paying a moment of attention to the Zabbix frontend configuration file. It's suggested to compare the old configuration file with the new one and see whether there're any new parameters or significant changes to the existing parameters. The easiest way might be comparing with the zabbix.conf.php.example file in the conf/ subdirectory. This configuration file is pretty small, so spotting the differences should be easy.