MC1287370 - Auto upgrade of shared calendars from legacy MAPI model to modern REST model

Updated April 29,May 6, 2026: We have updated the content. Thank you for your patience. 

Introduction

Microsoft is introducing an automatic upgrade for shared calendars in Outlook and Exchange Online, transitioning from the legacy MAPI-based model to the modern REST-based model. This server-side upgrade will improve calendar sync reliability, performance, and consistency for delegates and users who access shared calendars. The change is designed to be seamless and will not require any action from users or administrators.

When this will happen

General Availability (Worldwide):

Rollout starting early mid-May 2026 (previously early May) and completing by late July 2026

GCC High /DoD:

Rollout starting end of June 2026 (previously mid-May 2026May) and completing by late July 2026

Note: Rollout will be progressive and timing may vary by tenant.

How this affects your organization

Who is affected

• Microsoft 365 tenants using Exchange Online
• Users who have Editor or higher permissions on shared calendars in user or shared mailboxes
• Delegates and users who access shared calendars in Outlook

What will happen

• Shared calendars will be automatically upgraded from the legacy MAPI-based model to the modern REST-based model.
• A new calendar folder will be created in a hidden state and will only become visible to the user once the full sync is successfully completed and validated, ensuring a seamless rollover to the new model.
• The transition is expected to appear seamless, with no interruption to normal user workflows.
• Events older than one year will be excluded from the initial sync window. However, recurring events with instances falling within or after the one-year threshold will still be included in the sync.
• Existing permissions and sharing relationships will be preserved; no re-sharing or permission reset will be required.
• The upgrade process is fully automatic and requires no action from the user — no Outlook restart, profile recreation, or any other manual steps.
• No data loss is expected as a result of this change.
• The rollout will initially support auto-upgrade for up to four shared calendars per delegate or principal, scaling up to six over time. Those managing more than six shared calendars will be excluded from the auto-upgrade and will experience no change to their current setup.

How does it effect the Classic Outlook setting 'Turn on shared calendar improvements'?

• After migration, Classic Outlook will continue to honour the existing setting. Although shared calendars are upgraded to the REST model at the service level, Classic Outlook will follow the setting and use MAPI-based model if the setting is turned off. This behaviour is limited to Classic Outlook; all other endpoints (Outlook Web, Mobile, Mac, New Outlook) will use REST.

What you can do to prepare

No action is required.

We recommend that administrators:

• During migration, a hidden shared calendar folder may be visible to tenant admins and support teams via the Get-MailboxFolderStatistics cmdlet. This is expected and has no impact on end users.
• Monitor for persistent issues, such as missing meetings after the migration window or duplicate calendar folders, and contact Microsoft support if observed.
• If a manual rollback is required after the calendar becomes visible, open a support request through standard Microsoft support channels.

Compliance considerations

No compliance considerations identified. Review as appropriate for your organization.