Ending Exchange 2019/Exchange online hybrid question

Hi @Susan Dodds

Thanks for outlining your migration idea, it’s great that you’re planning ahead before making big changes. 

The approach you're describing, setting up a temporary hybrid, migrating, and then canceling the subscription isn't something that's supported and could cause some serious problems. The reason is that it breaks a fundamental link in how things work. Your Exchange Online mailboxes need a continuous identity connection provided by Entra ID (formerly Azure AD). 

• Loss of your data: Entra ID isn’t a separate paid service you rent, it’s a core part of your Microsoft 365 subscription. Canceling or allowing your Microsoft 365 tenant to expire will permanently delete all user accounts and their associated data (mailboxes, OneDrive files, etc.) after the retention period. 
• Loss of management: In a hybrid setup, your on‑premises Active Directory is the “source of truth” for user identities. Without the synchronization link, you lose the ability to manage important details, which can lead to issues with mail flow and administration. 
• Supported migration path: You can either maintain directory synchronization with Entra ID Connect after moving your mailboxes, or transition to a fully cloud‑only identity model. Both are supported depending on your long‑term business requirements. Once all mailboxes and services are confirmed to be working in the cloud, you can safely decommission and uninstall your on‑premises Exchange 2019 server. 

Following this approach ensures a secure, stable, and manageable environment while avoiding risks like data loss or administration issues. You can review the official Microsoft guidance here: How and when to decommission your on-premises Exchange servers in a hybrid deployment | Microsoft Learn 

I hope this helps clarify the process so you can plan a smooth migration. If you have any further questions, feel free to reach out. 

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