Elasticsearch can read indices created in the previous major version. If you have indices created in 5.x or before, you must reindex or delete them before upgrading to 7.9.1. Elasticsearch nodes will fail to start if incompatible indices are present. Snapshots of 5.x or earlier indices cannot be restored to a 7.x cluster even if they were created by a 6.x cluster. Any index created in 6.x is compatible with 7.x and does not require a reindex.
This restriction also applies to the internal indices that are used by Kibana and the X-Pack features. Therefore, before you can use Kibana and X-Pack features in 7.9.1, you must ensure the internal indices have a compatible index structure.
You have two options for reindexing old indices: