Only after step 6 of the article (Re-index all - which deletes the current indexes) is it mentioned to take a backup of the indexes:
Upgrading to Elasticsearch 7
Here, you'll learn to upgrade an existing Elasticsearch 6 server (or cluster) to Elasticsearch 7:
[... steps here]
6. Re-index all search and spell check indexes.
Before Proceeding, back up your existing data before upgrading Elasticsearch.
It would be more fortunate to suggest taking a backup before the steps or as the first step.