Release Notes
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Upgrading to Elasticsearch 5.6

There are a number of template changes in Elasticsearch 5.2, most notably around string type handling, that may cause issues when upgrading. If you are upgrading from Elasticsearch 2.x to Elasticsearch 5.6, you will need to re-index. For information on the type mapping changes, see the section called “Type Mapping Changes”.

For more information, see Upgrading Elasticsearch.