Catch 22 : downgrade to reindex : fails to start ES

We were running 7.9.1 but after after a machine power outage, ES did not start due to
an index created at version 5.6. And recommends downgrading to 6.x and reindex.

So I replaced with dpkg -i elasticsearch-oss-6.8.13.deb and
now I get an error message in startup :
failed to read [id:116, file:/dynga/es-iou2/elasticsearch/nodes/0/_state/

    [2020-11-04T10:41:59,031][WARN ][o.e.b.ElasticsearchUncaughtExceptionHandler] [] uncaught exception in thread [main]
org.elasticsearch.bootstrap.StartupException: ElasticsearchException[ failed to read [id:116, file:/dynga/es-iou2/elasticsearch/nodes/0/_state/]]; nested: IOException[failed to read [id:116, file:/dynga/es-iou2/elasticsearch/nodes/0/_state/]]; nested: XContentParseException[[-1:36] [node_meta_data] unknown field [node_version], parser not found];

should I also downgrade some or all of the suppor packages to do the reindexing necessary ?

    opendistro-alerting opendistro-anomaly-detection opendistro-index-management opendistro-job-scheduler opendistro-knn
  opendistro-performance-analyzer opendistro-security opendistro-sql opendistroforelasticsearch

I turns out that after trying a couple of other version og elasticsearch-oss the 7.9.1 version eventually accepted the index and came up. Case dismissed !