In order to upgrade chewy5/ES5 to chewy6/ES6 in the most seamless manner you have to:

• Upgrade to the latest 5.x stable releases: Chewy 5.2, ES 5.6
• Migrate any multi-typed indexes into single-typed
  • Using multi-index queries could be helpful
  • Parent/Child relationship is deprecated in favor of the join field
• Handle deprecation of string type & not_analyzed value for the index mapping parameter:
  • replace fields with { type: 'string', index: 'not_analyzed'} by {type: 'keyword'}
  • replace fields with { type: 'string', index: 'analyzed'} by {type: 'text'}
• PathHierarchy tokenizer' param delimiter now accepts only one argument, others should be replaced by character filter
• Make sure you don't use any other of the deprecated ES5 features
• Run your test suite on ES6
• Run manual tests on ES6
• Perform a rolling upgrade of Elasticsearch
• Upgrade to Chewy 6