The migration from elasticsearch to opensearch is estimated to run for many days, potentially a week or more. We will be in a mixed-cluster state for that time period. Historically, during major version upgrades, there have been limitations around things moving backwards. For example once a shard primary was on the new version it could not move back to an old version. Some similar limitations may exist here. We should run some tests to understand what limitations might be imposed by this.

We might also want to try and understand if we it matters when we migrate the master-capable nodes. First, last, or at any arbitrary time.

Incomplete list of potential tests:

• elastic can join cluster with opensearch master
• replica shard can move from opensearch to elastic
• primary shard can move from opensearch to elastic
• master can move from opensearch to elastic
• master can move from elastic to opensearch
• new primary shards can be allocated to elastic by opensearch master
• new primary shards can be allocated to opensearch by elastic master