What is a delta migration?
A delta migration copies only the items that are new or changed since the last copy ran. Delta means difference. Instead of moving every message and file again, the tool compares the source with what is already in Google Workspace and moves just the gap. It is the second half of a two-step approach: a bulk copy first, then one or more delta passes.
Why not copy everything on cutover day?
Copying everything on cutover day would leave your team without their full email history for as long as the copy takes, and large copies take time. Both Google and Microsoft limit how quickly data can move, so a mailbox with years of history cannot move in an instant. If you switch MX records first and copy later, people open Gmail to a nearly empty inbox and wait.
A delta approach avoids that. The slow part happens in the background, days or weeks before the switch, while people keep using the old system. The cutover itself only has to handle a small amount of recent mail.
How does a delta migration work step by step?
A delta migration works in a fixed sequence:
- Bulk copy. The tool copies mailboxes, contacts, calendars, and files from the old system into Google Workspace. People keep working as normal.
- Pilot check. A few users sign in to Google and confirm their history looks right.
- MX cutover. You change your domain's MX records so new mail goes to Gmail. Our guide to MX record cutover covers the timing.
- Delta pass. The tool copies anything that arrived or changed in the old system since the bulk copy, including messages delivered there while DNS updated.
- Verification. You confirm that recent messages and calendar changes appear in Google, then move the old system toward retirement.
Some projects run more than one delta pass, for example one shortly before the cutover and one after. Each pass is smaller than the last, because less has changed since the previous run.
What does a delta pass pick up, and what might it miss?
A delta pass picks up new messages, new calendar events, and new or changed files, depending on the tool and the source system. That covers the gap that matters most to your users. It is not a full mirror of every action, though. Tools differ in how they handle items that were deleted, moved between folders, or edited after the first copy, so test those cases before you rely on them.
A delta pass also does not move things that never migrate on their own, such as mail rules, signatures, and chat history. Our guide to what does not migrate automatically lists those so you can plan for them.
Does every migration need a delta pass?
Almost every migration with active users benefits from a delta pass. The exception is a very small move where everyone can stop using the old system during the copy. Even then, a delta pass is cheap insurance against a message that slipped through. Delta sync is included in our Standard and Premium plans.
It is also a big part of why the cutover can be short even when the copy is long. Our guide on how long a Google Workspace migration takes explains the rest of the timeline, and how to plan a Google Workspace migration shows where the delta pass fits in the full process.
Next step
Compare what each plan includes on our migration plans and process page, including delta sync on managed plans.
Frequently asked questions
How many delta passes does a migration need?
Usually one or two. A common pattern is one pass shortly before the cutover to shrink the gap, and one after the MX change to catch mail that arrived during propagation. Large or staged migrations may run more, often one for each batch of users.
Will users notice the delta pass?
Not in a way most people notice. Recent messages simply appear in Gmail alongside older mail once the pass completes. Users may see a short delay for messages sent during the switch, which is why the day-before message to staff should tell them to expect it.
