there are a few possibilites to go from an higher elastic version than 7.10 to opensearch.
- install a fresh graylog with opensearch on the side of the old one, and run both until no data is left on your old graylog due to data retention
- dump the elastic database and import it to opensearch / reindex the whole thing on opensearch
- get rid of the extra indices build by elastic which prevent the migration to opensearch. This is quite difficult.
all of them are painful, but I have not found a better solution. :-/