The current README states that reindexing works like this:

1. Create a new index with a "reindexed-" prefix (#30114).
2. Create an index alias pointing from the original index name to the prefixed index name.
3. Reindex from the original index into the prefixed index.
4. Delete the old index and rename the prefixed index.

Per @yuliacech's comment at #120137 (comment), the correct steps are:

1. Set index test_data to read-only using the index.blocks.write: true setting
2. Create new index reindexed-v6.8-test_data
3. Reindex documents from test_data to reindexed-v6.8-test_data
4. Create alias test_data that points to reindexed-v6.8-test_data
5. Delete index test_data

Whoever updates the README should read through https://github.com/elastic/kibana/blob/main/x-pack/plugins/upgrade_assistant/server/lib/reindexing/reindex_service.ts and verify what I've stated is correct.

While we're at it, we should also update this comment:

The comment states "so new data can be indexed", but that doesn't make sense because we're setting the index to be read-only. This might have just been a typo.