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 Elasticsearch Add-On for Heroku console 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