A known issue can prevent direct rolling upgrades from Elasticsearch version 5.6.10 to version 6.3.0. As a workaround, we have removed version 6.3.0 from the Cloud UI for new cluster deployments and for upgrading existing ones.
If you are affected by this issue, see Rolling upgrades from 5.6.x to 6.3.0 fails with "java.lang.IllegalStateException: commit doesn’t contain history uuid" in our Elastic Support Portal. If these steps do not work or you do not have access to the support portal, you can contact firstname.lastname@example.org.
The issue will be fixed in version 6.3.1 of the Elastic Stack.
Service update: June 25, 2018