Delta sync: how to run a low-disruption M365 cutover

The traditional big-bang migration — freeze source, migrate everything, cut DNS — means hours of downtime and panicked users. Delta sync is the alternative: you migrate the bulk of data days or weeks before cutover, then run quick incremental passes to catch up, so the final DNS flip takes minutes not hours.

The three-pass strategy

1. 1Initial sync — run 1–2 weeks before cutover. Migrates all existing email, files, and calendar data. Takes the longest but happens while users are still active on the source.
2. 2Delta sync — run 24–48 hours before cutover. Only migrates items created or modified since the initial sync. Much faster.
3. 3Final delta — run in the cutover window, immediately before flipping DNS. Catches the last few hours of changes.

What delta sync tracks

For email, delta sync uses Microsoft Graph's delta query endpoints, which return only items that have changed since a given delta link was issued. For SharePoint and OneDrive, it uses the driveItem delta endpoint. Both give you a precise list of created, modified, and deleted items since the last run.

Planning the cutover window

With a three-pass strategy, the cutover window only needs to cover:

• The final delta sync (typically 15–60 minutes depending on activity since the last pass).
• DNS propagation — lower your MX and Autodiscover TTL to 300 seconds at least 48 hours before cutover.
• User profile reconfiguration — Outlook Autodiscover should handle this automatically if DNS is correct.

A well-planned delta sync cutover can be achieved in a 2-hour window, most of which is DNS propagation. The actual migration work is often under 30 minutes.

Teams and SharePoint considerations

Teams message history doesn't support delta sync — the Teams Migration API imports messages by timestamp, not by change detection. The practical approach is to do a final full Teams channel sync in the cutover window, which is fast if the initial sync already copied the bulk of history.