Is there a way to make elasticsearch case-insensitive without altering the existing documents?

By Default, the fields are case-insensitive because of the mapping elastic applied.

Try below:

PUT myindex/doc/1{  "name":"TEST"}GET myindex/_mapping

It should return :

{  "myindex": {    "mappings": {      "doc": {        "properties": {          "name": {            "type": "text",            "fields": {              "keyword": {                "type": "keyword",                "ignore_above": 256              }            }          }                  }      }    }  }}

Now if you query with below, it will return a match (notice the mapping[text and keyword]):

POST myindex/_search{  "query": {    "match": {      "name2": "test"    }  }}

Now, if you explicitly specify to index the field as keyword, then it will be case-sensitive search. Try below and see; it will not return any results.

PUT myindex/_mapping/doc{  "properties": {    "name2": {      "type": "keyword"    }  }}PUT myindex/doc/1{  "name2":"TEST"}POST myindex/_search{  "query": {    "match": {      "name2": "test"    }  }}

TLDR; Use default mapping or text type- if you specify the field to index only keyword type, it will be case-sensitive.