We're currently running Elasticsearch 6.2.4 (about to be 6.4.3 following AO3-5547 ).
We'd like to be running Elasticsearch 6.5 latest. james_ will need to update the version on the servers, but we can make any necessary changes to the codebase. These are the breaking changes for 6.5.
To test, we’ll want to give searching and filtering a quick poke on staging to make sure everything is normal. There are no code changes, just test changes, so there is no particular area to poke.