How to upgrade elastic search 2.4.1 to 7.3.2 without affecting the index ?
Upgrading two major versions will require you to change mappings and reindex your data.
https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-snapshots.html
But in this link, it has been mentioned that "snapshots of indices created in 2.x cannot be restored to 6.x or 7.x"
Exactly, you will need to create updated mappings or index template and then reindex from remote. Snapshots will not work.