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  "description": "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, a...",
  "path": "/m365-message-center/message/mc1287370/",
  "publishedAt": "2026-04-22T00:00:07.000Z",
  "site": "https://blog.tophhie.cloud",
  "textContent": "**[Introduction]**\n\nMicrosoft 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.\n\n**[When this will happen]**\n\n**General Availability (Worldwide):**\n\n  * Rollout start: **Late April 2026**\n  * Rollout completion: **Late July 2026**\n\n\n\nRollout timing may vary by tenant.\n\n**[How this affects your organization]**\n\n_Who is affected_\n\n  * Microsoft 365 tenants using Exchange Online\n  * Users who have **Editor or higher permissions** on shared calendars in _user or shared mailboxes_\n  * Delegates and users who access shared calendars in _Outlook_\n\n\n\n _What will happen_\n\n  * Shared calendars will be **automatically upgraded** from the legacy MAPI-based model to the modern REST-based model.\n  * The service will **create a new calendar folder** in a _hidden state_ , complete a **full synchronization** , and then **make the calendar visible**.\n  * The transition is expected to appear seamless, with no interruption to normal user workflows.\n  * **Events older than one year** will **not be included** in the initial sync window.\n  * **Existing permissions and sharing relationships** will be **preserved** ; no re-sharing or permission reset will be required.\n  * **No Outlook restart** , profile recreation, or user action will be required in _normal upgrade scenarios_.\n  * **No data loss** is expected as a result of this change.\n  * **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.\n\n\n\n**[What you can do to prepare]**\n\nNo action is required.\n\nWe recommend that administrators:\n\n  * Inform helpdesk and support teams that **temporarily hidden shared calendars during migration are expected** and do not indicate a failure.\n  * **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.\n  * Monitor for persistent issues, such as missing meetings after the migration window or duplicate calendar folders, and contact Microsoft support if observed.\n  * If a manual rollback is required after the calendar becomes visible, open a support request through standard Microsoft support channels.\n\n\n\n**[Compliance considerations]**\n\nNo compliance considerations identified. Review as appropriate for your organization.",
  "title": "MC1287370: Auto upgrade of shared calendars from legacy MAPI model to modern REST model",
  "updatedAt": "2026-04-22T00:00:07.737Z"
}