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

Microsoft will auto-upgrade shared calendars in Exchange Online from the legacy MAPI model to the modern REST model between April and July 2026. The seamless, server-side upgrade improves sync reliability without user action, preserves permissions, excludes events older than one year initially, and requires no Outlook restart. 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 […]

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