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

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 start: Late April 2026
• Rollout completion: Late July 2026

Rollout 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.
• The service will create a new calendar folder in a hidden state, complete a full synchronization, and then make the calendar visible.
• The transition is expected to appear seamless, with no interruption to normal user workflows.
• Events older than one year will not be included in the initial sync window.
• Existing permissions and sharing relationships will be preserved; no re-sharing or permission reset will be required.
• No Outlook restart, profile recreation, or user action will be required in normal upgrade scenarios.
• No data loss is expected as a result of this change.
• Up to four shared calendars per delegate or per principal will be eligible for upgrade during this rollout cycle and will slowly be upgraded to 6.

What you can do to prepare

No action is required.

We recommend that administrators:

• Inform helpdesk and support teams that temporarily hidden shared calendars during migration are expected and do not indicate a failure.
• Do not recommend deleting and re-adding shared calendars, toggling Shared Calendar Improvements, or creating new Outlook profiles during the migration, as temporary changes may occur while the upgrade completes.
• 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.