How to fix IllegalArgumentException: mapper [field] cannot be changed from type [text] to [keyword] in Elasticsearch

First, verify the current field type in your index mapping:

curl -X GET "localhost:9200/your-index/_mapping/field/your-field-name?pretty"

Or for the entire index mapping:

curl -X GET "localhost:9200/your-index/_mapping?pretty"

This will show you what type the field is currently mapped as, confirming the conflict.

Create a new index with the desired field type. First, get the current mapping and modify the problematic field:

curl -X PUT "localhost:9200/your-index-v2" -H "Content-Type: application/json" -d '{
  "mappings": {
    "properties": {
      "your-field-name": {
        "type": "keyword"
      },
      "other-field": {
        "type": "text"
      }
    }
  }
}'

Replace "your-index-v2" with your new index name, and ensure all fields are defined with their correct types. Copy the full mapping from the old index and change only the fields that need type corrections.

Use Elasticsearch's Reindex API to copy all data from the old index to the new one, applying the new mapping:

curl -X POST "localhost:9200/_reindex" -H "Content-Type: application/json" -d '{
  "source": {
    "index": "your-index"
  },
  "dest": {
    "index": "your-index-v2"
  }
}'

This operation copies only the raw data and applies the destination index's mapping. The process may take time for large indices. Monitor progress with:

curl -X GET "localhost:9200/_tasks?detailed=true&actions=*reindex*"

After reindexing completes, verify that documents were copied correctly and the field type is now correct:

curl -X GET "localhost:9200/your-index-v2/_count?pretty"
curl -X GET "localhost:9200/your-index-v2/_mapping?pretty"

Check the document count matches the original index, and confirm the field now has the desired type in the mapping.

Once verified, update your application to point to the new index. If you're using aliases, update them:

curl -X POST "localhost:9200/_aliases" -H "Content-Type: application/json" -d '{
  "actions": [
    { "remove": { "index": "your-index", "alias": "your-index-alias" } },
    { "add": { "index": "your-index-v2", "alias": "your-index-alias" } }
  ]
}'

This way, your application continues using the same alias name while seamlessly pointing to the new index.

Once you've confirmed everything works with the new index and your application is using it, you can delete the old index to free up storage:

curl -X DELETE "localhost:9200/your-index"

Only do this after you're completely confident the migration is successful.