Upgrading from Elasticsearch 1.x to 2.x involves several critical changes that require careful planning to ensure a smooth transition. The process mandates a full cluster restart and involves familiarizing oneself with the breaking changes detailed in the new release documentation. Key steps include setting the network host correctly in the elasticsearch.yml file, utilizing the migration plugin to identify necessary changes, and leveraging the snapshot and restore feature to test the upgrade in a non-production environment. It is essential to upgrade associated tools like Logstash and Kibana to compatible versions, with specific attention to plugin compatibility and configuration adjustments. In the event of issues, users are encouraged to seek support through community forums or customer support channels. For those seeking to minimize downtime, an alternative approach is to build a new cluster on the updated version, ensuring all data and functionalities are transferred before decommissioning the old system. Known issues, such as plugin compatibility and field mapping conflicts, must be addressed to prevent upgrade failures.