Upgrade ElasticSearch to Opensearch taking 2 months

Hi there, in general a blue/green deployment process will be used when upgrading domains and cluster state would change to "Processing" while a new fleet of nodes is launched with the new configuration. There could be many reasons that will cause cluster to be stuck in processing. Some of the reasons could be:

1. The cluster is overloaded (high CPU and JVM or low free storage).
2. Large number of shards significantly more than recommended.
3. Cluster is in red state or node out of service.
4. Making simultaneous configuration changes on cluster.

If none of these apply in your case as the update is stuck for so long, the best next steps for you would be to open a Support case as this might need assistance from AWS OpenSearch team to retry the process. Sorry it would be difficult to provide an appropriate solution without looking into logs and other detailed information.