The day has finally arrived: The Exchange Team released Exchange Server Subscription Edition, or SE for short. The official announcement can be found here. Customers keeping Exchange on-premises or who are running Exchange hybrid deployments are recommended to use the remaining time this year to upgrade to SE before their current supported Exchange server, being Exchange 2016 or 2019, goes out of support in October.
Exchange Server SE has feature parity with Exchange Server 2019 CU15, meaning it contains no changes in features or security posture. Significant change Exchange SE introduces is a change of servicing and (new) lifecycle period, also known as Modern Lifecycle Policy. In essence, products have no end-of-life date provided that customers keep their products updated. Contrary to earlier Exchange versions, this means the product must be kept current, and the n-2 rule, meaning organizations could be trailing one update, will no longer apply.
Co-existence
In a nutshell, Exchange SE RTM can be installed in organizations running Exchange 2016 or Exchange 2019. Servers running Exchange 2019 CU14+ can be in-place upgraded to Exchange SE by installing SE over the current build, as if it were a Cumulative Update. It does not require any schema or Active Directory changes; it just changes the product name, license agreement (modern lifecycle policy), and build numbers. SE also incorporates the May 2025 hotfix. An additional benefit is that it does not temporarily require twice the resources to move, unlike Exchange 2016, which basically consists of a classic mailbox migration. Lastly, More on the upgrade path here.
Post-RTM
When support for Exchange 2016 and Exchange 2019 ends in October this year, Exchange Server SE will be the only Exchange on-premises product that is supported. While these old Exchange versions will not suddenly stop functioning, Exchange SE CU2 will block co-existence with Exchange 2016 and Exchange 2019. This means you only have from now until the arrival of Exchange SE CU2 to upgrade. Future Exchange SE CUs will introduce new features and may start requiring Exchange SE keys when hosting mailboxes.