Copy_to for the existing data not new coming one

I am pretty new to Elasticsearch.

Now I am trying to upgrade my cluster for the matter of better performance and functionalities, in particular, aggregation pipelining feature.

In order to run a proper aggregation, I would like to use copy_to option rather than script.
(My problem is, the data I want to aggregate is in several different fields)

I followed the documentation here for copy_to and here for updating _mapping.

However, it seems the copy_to affects only new data but not for existing one.

I run curl -XPUT http://localhost:9200/_mapping/type?pretty=true -d '{ "properties" : { "fieldA" : { "type" : "string" , "copy_to": "fieldB" } } }' and checked the mapping is updated as below.

{"properties" : {
          "fieldA" : {
            "type" : "string",
            "copy_to" : "fieldB"

Am I missing something?

Is there any clever way to copy data in existing field to another field?

I think I have no choice but reindexing here.

Please share if you have any good idea. Thanks.

You need to reindex if you want this to change for older data.