Hi,
If your only intension is to add new field to the existing documents, then update API is the best way to do.
https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-put-mapping.html
https://divyanij.medium.com/adding-a-new-field-in-an-existing-index-elasticsearch-a3cf25c053fb
But, if you want to delete complete data and ingest data freshly again, then my suggestion is to create data using new index names so that your old data can be deleted once you feel everything is fine in the new index.
Reindexing is used to copy data from one source to another destination.
https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html