Elasticsearch Mapping - Rename existing field

You could do this by creating an Ingest pipeline, that contains a Rename Processor in combination with the Reindex API.

PUT _ingest/pipeline/my_rename_pipeline{  "description" : "describe pipeline",  "processors" : [    {      "rename": {        "field": "fieldCamelcase",        "target_field": "fieldCamelCase"      }    }  ]}POST _reindex{  "source": {    "index": "source"  },  "dest": {    "index": "dest",    "pipeline": "my_rename_pipeline"  }} 

Note that you need to be running Elasticsearch 5.x in order to use ingest. If you're running < 5.x then you'll have to go with what @Val mentioned in his comment :)

Updating field name in ES (version>5, missing has been removed) using _update_by_query API:

Example:

POST http://localhost:9200/INDEX_NAME/_update_by_query{  "query": {     "bool": {        "must_not": {            "exists": {                "field": "NEW_FIELD_NAME"            }        }    }  },  "script" : {    "inline": "ctx._source.NEW_FIELD_NAME = ctx._source.OLD_FIELD_NAME; ctx._source.remove(\"OLD_FIELD_NAME\");"  }}

First of all, you must understand how elasticsearch and lucene store data, by immutable segments (you can read about easily on Internet).

So, any solution will remove/create documents and change mapping or create a new index so a new mapping as well.

The easiest way is to use the update by query API: https://www.elastic.co/guide/en/elasticsearch/reference/2.4/docs-update-by-query.html

POST /XXXX/_update_by_query{    "query": {     "missing": {      "field": "fieldCamelCase"    }  },    "script" : {        "inline": "ctx._source.fieldCamelCase = ctx._source.fieldCamelcase; ctx._source.remove(\"fieldCamelcase\");"    }}