Zabbix agents from previous Zabbix versions are compatible with Zabbix 3.4. However, you may need to review the configuration of older agents as some parameters have changed, for example, parameters related to logging for versions before 3.0.
To take full advantage of new and improved items, improved performance and reduced memory usage, use the latest 3.4 agent.
Both Zabbix 3.4 proxies and Zabbix 3.4 server are supported to work only with Zabbix 3.4 server and Zabbix 3.4 proxies respectively.
It is known to be possible to start an upgraded server and have older, yet unupgraded proxies report data to a newer server (the proxies can't refresh their configuration though). This approach, however, is not recommended and not supported by Zabbix and choosing it is entirely at your own risk. For more information, see the upgrade procedure.
Zabbix 3.4.10 server can only work with Zabbix 3.4.10 proxies. Compatibility with proxies from other 3.4.x versions has been lost. This issue is fixed for Zabbix server 3.4.11.
XML files, exported with 1.8, 2.0, 2.2, 2.4, 3.0 and 3.2 are supported for import in Zabbix 3.4.
In Zabbix 1.8 XML export format, trigger dependencies are stored by name only. If there are several triggers with the same name (for example, having different severities and expressions) that have a dependency defined between them, it is not possible to import them. Such dependencies must be manually removed from the XML file and re-added after import.