Hey @totemz
Can I ask how you configured your index set? Also someone had a simialer issue with graylog config file. Ther eupgrade wrote over the old file shown here
Check you configration again make sure the path.data: /var/lib/opensearch
is correct.
Here are couple more commands for troubleshooting
#### See if you can find the old index set's ###
curl -XGET http://localhost:9200/_cat/indices?v
curl -XGET http://localhost:9200/_cluster/allocation/explain?pretty