Mesosphere only supports DC/OS upgrades from a General Access (GA) release of DC/OS to another GA release of DC/OS currently. Any upgrades involving (to/from) an Early Access (EA) are not supported
We have invested alot of engineering time to ensure upgrades from any DC/OS GA release to the newest GA release is as smooth as possible. Typically this means upgrades are initiated from one major release to another.....for example, 1.7.4 to 1.8.4 or1.7.4 to 1.8.8.
We do not recommend customers to upgrade their production or critical clusters from GA to EA release for obvious concerns around stability. EA releases are run continuously with significant workloads internally at Mesosphere to mimic real customer workloads in an effort to uncover production impacting defects. Only when these production impacting defects are identified and fixed for long running workloads does a DC/OS release candidate (EA) receive the GA release status. At this point DC/OS is made available to the Community and Enterprise customers.
Since there has not been a stable or GA version of DC/OS 1.9, upgrades are not officially supported yet.
For more information on our Versioning and Release Policy, please see https://docs.mesosphere.com/1.8/version-policy/