migrationonboardingcalendar

How to Replace Microsoft 365 Calendars Without Breaking Your Scheduling Workflows

ccalendarer

2026-02-10

10 min read

A 2026 migration playbook to move off Microsoft 365 calendars—preserve invites, integrations, and minimize downtime.

Stop losing hours to calendar chaos: a migration playbook that preserves meetings, invites, and integrations

Switching away from Microsoft 365 is more than a software choice—it's a workflow migration. Operations teams worry about lost invites, broken booking pages, and integrations that silently stop working. This 2026 playbook gives business buyers and small operations teams a practical, step-by-step plan to replace Microsoft 365 calendars (for example, when moving to LibreOffice for documents plus a CalDAV-backed calendar like Nextcloud) while keeping meetings, invites, and integrations intact and minimizing downtime.

Why this matters now (2026 context)

In late 2024–2025, many organizations revisited Microsoft 365 licensing and data-control tradeoffs. By 2026, two clear trends drive calendar migrations:

Cost and vendor diversification: sustained license pressure pushed SMBs and public sector teams to evaluate open-source alternatives and modular stacks.
Privacy-first and federation-friendly tooling: post-2024 regulation and procurement changes increased interest in CalDAV/ICS-based groupware and self-hosted options.

LibreOffice is an excellent drop-in replacement for documents, but it is not a calendar server. For a credible Microsoft 365 replacement, pair LibreOffice with a groupware calendar backend (Nextcloud, Zimbra, Fastmail, or a hosted CalDAV service) and a standards-first approach (iCal/ICS, CalDAV, iTIP/iMIP). This article shows how to make that swap without breaking scheduling workflows.

High-level migration playbook

Follow these phases. Each phase includes practical steps, tools, and risks to watch.

Prepare & Audit
Export calendars & free/busy data
Choose new calendar backend & architecture
Import events and preserve metadata
Reconnect integrations and booking flows
Switch free/busy, resources, and sharing
Staged cutover to minimize downtime
Post-migration verification and training

Phase 0 — Prepare & Audit

Inventory users, shared calendars, room/resources, delegates, and third-party integrations (CRMs, booking tools, Zoom/Teams, payment/booking pages).
Identify high-risk calendars: recurring client meetings, legal holds, public-facing booking pages, and automations sending invites.
Set success metrics: acceptable downtime window, percentage of events retained with original UIDs, and integrations reconnected within X hours.
Create a pilot group (5–15 power users) and a rollback plan.

Phase 1 — Exporting calendars safely

Goal: get reliable copies of calendar data with attendees, recurrence rules, and attachments.

Recommended export methods

Outlook desktop export: File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst). This preserves attendee relationships and recurrence. Use this for full mailbox exports when you’ll import into a client that can read PST.
ICS/ICAL export: For single calendars or shared calendars, use the Outlook Web App (OWA) or the calendar export features to produce .ics files. ICS preserves event UIDs and recurrence rules in a standards-compliant way.
Admin-level export: For enterprise migrations use vendor tools (MigrationWiz, CodeTwo) or an admin export workflow. For Exchange Online, admins can also use eDiscovery exports to pull mailbox content including calendar items.
API-driven export: Use Microsoft Graph API to extract events in JSON and recompose to iCalendar if you need automation. This is ideal for large or scripted migrations.

Risk & mitigation: ICS export can lose some Exchange-specific metadata (like some reminders or custom attendee types). Export both PST and ICS where possible for safety.

Phase 2 — Choose a calendar backend and pairing for LibreOffice

Options in 2026 that pair well with LibreOffice:

Nextcloud (hosted or self-hosted) — strong CalDAV support, good app ecosystem for reminders and workflows; pairs with Collabora for collaborative editing of LibreOffice-compatible documents.
Zimbra or Kolab — full groupware including mail and calendar for organizations who want an Exchange-like stack without Microsoft.
Hosted CalDAV providers (Fastmail, mailbox.org) — low management overhead and reliable mobile/desktop sync.
Google Workspace — if you prefer cloud but not Microsoft; note differences in export/import semantics.

Decision factors: number of users, self-hosting capability, SSO needs, resource scheduling, and how many external integrations you must reconnect.

Phase 3 — Importing calendars and preserving metadata

Best practice: import on the server side so events are visible to all clients and free/busy is accurate immediately.

For ICS imports use server import utilities (Nextcloud offers a server-side calendar import for admins; many groupware servers accept bulk ICS uploads).
Preserve original UID and SEQUENCE fields where possible. This reduces duplicate events and helps clients recognize updates.
When you can't preserve UID, map old UIDs to new IDs in a migration log to support later update reconciliation. Server-side imports are easier to verify with your monitoring and operational dashboards.

Phase 4 — Preserving invites, attendees, and meeting links

Invites use iTIP/iMIP standards; keep the attendee list intact and send an update where necessary:

If you imported events with attendees intact, send a controlled meeting update from the new server notifying attendees of the system change. This keeps the thread consistent and avoids duplicate confusion.
For meetings that include Microsoft Teams links, plan how to migrate or replace those conferencing links: replace Teams links with Zoom/Google Meet/Jitsi links and include instructions in the updated invite.
Avoid mass re-sends that look like spam. Time updates to off-peak windows and mark them as “system update” with a short note.

Phase 5 — Reconnecting integrations and booking flows

Every third-party integration that reads/writes calendars needs reauthorization and testing.

Booking tools (Calendly, Acuity, custom booking pages): swap calendar connections to the new CalDAV/ICS endpoint or use the provider's API to configure a new calendar. Update webhooks and confirmation email templates.
CRM and ticketing systems: reconfigure calendar sync adapters. If the CRM only supports Exchange APIs, use an adapter or middleware that translates between CalDAV and the CRM's expected interface — see guides on legacy-system migration for patterns that work.
Meeting platform connectors (Zoom, Webex): re-register OAuth tokens and test that meeting creation creates the right conferencing links.

Phase 6 — Free/busy, rooms, and resource scheduling

Free/busy data is the glue of team scheduling.

Set up server-side free/busy publishing or federation. CalDAV servers typically provide free/busy endpoints; configure clients and shared calendars to query those endpoints. Test free/busy endpoints and log differences to your monitoring dashboards.
Recreate room/resource calendars and set the same access controls as before (booking permissions, auto-accept, delegates).
Test with 3–5 sample meetings that involve cross-organization scheduling to confirm free/busy behavior; verify that your export/import preserved timestamps and data governance expectations.

Phase 7 — Staged cutover to minimize downtime

Zero-downtime is possible with a dual-write and staged cutover approach.

Keep both systems running in parallel for a defined transition window (2–14 days depending on scale).
For incoming invites: use forwarding rules or middleware to copy incoming calendar emails to the old and new systems.
Perform a final delta export/import immediately before the cutover to capture last-minute changes.
Change DNS, update MX/SMTP routing only when you are ready to short-circuit new invites directly into the new system.

Risks: email-based invites that create events automatically can create duplicates if middleware is misconfigured. Maintain a migration log to reconcile duplicates quickly.

Phase 8 — Post-migration verification and training

Run verification scripts: compare event counts, check UID mapping integrity, and test integration webhooks; export logs for postmortem analysis and reporting to stakeholders.
Enable an incident window (72 hours) where the migration team is on-call to remediate missed meetings and client-facing errors.
Train staff on new client apps (Thunderbird/Lightning, mobile CalDAV apps, Nextcloud calendar UI) and provide a one-page “how to accept an updated invite” quick-reference.

Technical deep-dive: practical tips and examples

Exporting reliably from Outlook (quick guide)

Use the Outlook desktop client: File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst). Include calendar folders.
Create an .ics export for public/shared calendars from OWA when you need a standards-compliant calendar file.
If you manage many mailboxes, use a migration tool (MigrationWiz, CodeTwo) or an admin-level eDiscovery export to avoid manual steps.

Importing into Nextcloud/CalDAV server (tips)

Use Nextcloud’s server import for bulk ICS; verify UID preservation in the resulting calendar items.
For resource calendars, create the resource first and import ICS into it to keep permissions correct.

Handling recurring events and exceptions

Recurring events have complex rules and exceptions. Follow these rules:

Preserve RRULE, EXDATE, and exception event UIDs. If your import tool drops exceptions, you’ll end up with duplicated occurrences.
Test a sample of 50 complex recurring events (legal retainers, weekly sales calls) to validate exception handling before bulk import.

Integrations: OAuth tokens, webhooks, and API mappings

List all OAuth apps authorized in Microsoft tenants. Recreate these authorizations against the new provider and update redirect URIs.
Webhooks: update callback endpoints and test for event create/update/delete events.
For CRMs with Exchange-only connectors, use an adapter service that exposes a CalDAV-compatible endpoint or switches calendar synchronization to IMAP+CalDAV.

Case study: 75-person consultancy—practical example

Situation: a 75-person consultancy wanted to stop Microsoft 365 for cost reasons and move to LibreOffice for docs plus Nextcloud for calendaring. Key constraints: client meetings scheduled months in advance, a public booking page for consultations, and Zoom links embedded in invites.

Execution summary:

Pilot with 8 power users for 14 days to validate ICS import and resource calendar behavior.
Exported calendars as .pst and .ics, then imported server-side into Nextcloud. Preserved UIDs where possible and recorded mapped IDs for reconciliation.
Reconnected booking page to Nextcloud calendar via Calendly’s CalDAV integration and reissued booking confirmation templates with new meeting links.
Cutover weekend: last delta export on Sunday night, DNS cut Monday 01:00 local, and IT team on-call till Monday 17:00. Final reconciliation reduced duplicates by using UID mapping logs.

Outcome: no client meetings missed, booking pages remained functional, and the company regained full control of document storage with LibreOffice + Collabora. The firm also implemented automated SMS reminders (via Nextcloud Flow) and reduced no-shows within 60 days.

"A careful pilot, server-side imports, and a short dual-run period are the three elements that protect meetings and customer-facing links during calendar migration."

2026 trends and how to future-proof your calendar stack

Look for these developments and make migration decisions that survive them:

Federated calendars and improved CalDAV implementations: expect better cross-vendor free/busy queries and federation in 2026–2027—choose a provider that invests in standards.
API-first scheduling: more calendar providers offer granular webhooks and event-source APIs—this simplifies integrations and reduces fragile polling setups. Consider how your apps will integrate with API-first providers.
AI-assisted scheduling: modern schedulers will offer AI assistants that respect privacy constraints—ensure your calendar backend exposes secure APIs for assistant integration.
Privacy and data locality: regulations will continue to favor providers that can demonstrate local data storage and auditability. If you need compliance help, see migration guides and audits for sovereign-cloud moves.

Quick migration checklist (printable)

Inventory calendars, resources, and integrations.
Export PST and ICS backups for all users.
Choose target backend and validate CalDAV/ICS behavior.
Pilot import with power users and test recurring/exception events.
Reconfigure booking tools and recreate conferencing links.
Stage cutover with a dual-write period; perform a final delta import before switch.
Monitor first 72 hours and enable an incident window.

Common pitfalls and how to avoid them

Assuming LibreOffice equals calendar service — LibreOffice is for documents; pair it with a calendar server.
Relying only on client-side exports — server-side imports reduce duplication and speed propagation to all devices.
Not testing recurring exceptions — these break more often than simple single-instance events.
Forgetting integrations — audit and test every CRM, booking page, and conferencing connector before cutover.

Actionable takeaways

Plan for UIDs: preserve or map event UIDs to avoid duplicates and allow updates to flow.
Server-side imports: import calendars on the new server to make events immediately available to all clients and ensure free/busy accuracy.
Pilot and dual-run: validate with power users and keep both systems running for a short window to capture deltas safely.
Reauthorize integrations: update OAuth and webhook endpoints as part of the migration playbook and test each integration.

Next steps — get migration help

If you're planning a move in 2026, start with a short, fast audit that lists your calendar dependencies and a pilot plan. For operations teams that prefer to avoid scripting and manual reconciliation, professional migration services (or our migration checklist and hands-on support at calendarer.cloud) can save weeks of effort and prevent customer-facing mistakes.

Ready to migrate? Download our free migration checklist or request a migration audit to map calendars, integrations, and a zero-downtime cutover plan tailored to your business.

calendarer

Contributor

Senior editor and content strategist. Writing about technology, design, and the future of digital media. Follow along for deep dives into the industry's moving parts.