I have such error when trying different configuration parameters for ES at GCP:
org.elasticsearch.bootstrap.StartupException: java.lang.IllegalStateException: cannot downgrade a node from version [7.3.0] to version [7.2.0].
Is it mentioned in the documentation?
It is a kind of surprise for me although I believe 7.3.0 is better than 7.2.0.
You must have a snapshot of your data to roll back to an earlier version.
EDIT: In #47342 these docs were clarified so the link above no longer contains this sentence. Instead this page and this page both say this:
Once you have started to upgrade your cluster to version 7.4.0 you must complete the upgrade. As soon as the cluster contains nodes of version 7.4.0 it may make changes to its internal state that cannot be reverted. If you cannot complete the upgrade then you should discard the partially-upgraded cluster, deploy an empty cluster of the version before the upgrade, and restore its contents from a snapshot.
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