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APPLIES TO: 
2016 2019 Subscription Edition
Exchange Server Subscription Edition (SE) is capable of operating alongside earlier versions of Exchange Server within the same organization. Previously, organizations could run older, including unsupported, versions of Exchange Server together with newer versions. There are now two significant changes to this practice:
- The installation process for Exchange Server SE Release to Manufacturing (RTM) prevents coexistence with Exchange Server 2013.
- The setup procedure for Exchange Server SE CU2 will prohibit coexistence with any version of Exchange Server that is not supported at the time of release.
To upgrade Exchange Server 2016 or Exchange Server 2019 to Exchange Server SE, two upgrade methods are available:
- Legacy upgrade: This method involves transitioning to a new major version of Exchange Server by adding the newer server to the organization, migrating all mailboxes and resources from existing servers to the new servers, and then uninstalling the previous servers. Legacy upgrades are necessary when moving from Exchange Server 2016 to Exchange Server 2019 or to Exchange Server SE, as well as when switching to new hardware or a newer version of Windows Server.
- In-place upgrade: Exchange Server SE supports an in-place upgrade from Exchange Server 2019 Cumulative Update (CU) 14 or 15. This process allows you to install Exchange Server SE over your existing Exchange Server 2019 installation, similar to the procedure for applying cumulative updates. Please adhere to the recommended steps and best practices outlined in the Upgrade Exchange to the latest Cumulative Update documentation.
The Exchange Server Deployment Assistant guides you through the correct steps for your specific upgrade scenario. It generates a tailored, step-by-step checklist to help you successfully deploy Exchange Server SE in your on-premises environment.
For more context, see the blog posts Upgrading your organization from current versions to Exchange Server SE and Why in-place upgrade from Exchange 2019 to Exchange SE is low risk.
Exchange Server SE Product Key requirements
Exchange Server SE RTM does not require a new product key. It will continue to function after a legacy or in-place upgrade to Exchange Server SE. However, starting with a future cumulative update, Exchange Server SE will introduce a new product key requirement. This documentation will be updated with the latest requirements when they become available.