Moving to Microsoft 365 without downtime: what to expect

A well-run Microsoft 365 migration is mostly invisible to staff: one day they’re using the old system, the next they sign in to the new one and their mail, calendars and files are there. Getting there takes planning. Here’s how a typical small-business migration runs.

1. Inventory and planning

Before anything moves, list what you have:

• Mailboxes, shared mailboxes, distribution lists and aliases
• Approximate mailbox and file sizes
• Where files live today (file server, another cloud service, personal drives)
• Devices and phones that connect to email
• Applications that send email, such as scanners, line-of-business software and website forms
• Your domain’s DNS host and who can change it

This inventory determines the migration method, the timeline and the licences you need.

2. Choose licences

Microsoft 365 plans differ in security features, device management and desktop apps. The cheapest plan isn’t always cheapest overall if you then buy separate tools for features a business plan includes. Match the plan to how each user works.

3. Prepare and secure the tenant first

Configure the new environment before any data arrives: verify your domain, set up multi-factor authentication, block legacy sign-in methods, define admin roles, set sharing defaults and plan the SharePoint and Teams structure. Securing an empty tenant is far easier than securing one that’s already in use.

4. Pre-stage the data

Mailbox and file data is copied in the background while everyone keeps working on the old system. Larger mailboxes and file sets take longer, which is why pre-staging starts days or weeks before cutover.

5. Cutover

At a scheduled time — often an evening or weekend — mail delivery is switched by updating your domain’s MX records, a final synchronisation copies the last changes, and users start signing in to Microsoft 365. Outlook profiles and phones are reconfigured.

6. Day-one support

Expect a few questions: a missing shared calendar, a phone that needs re-adding, a scanner that needs new mail settings. Planning for this support is part of a smooth migration.

7. Verify, then decommission

Keep the old system available, read-only, until mail flow, data and devices are confirmed. Then decommission it, cancel old services, and set up an independent backup of Microsoft 365 data.

What doesn’t migrate automatically

• Some mailbox rules and client-side settings
• Delegated permissions on calendars and shared mailboxes, which may need re-applying
• Applications and devices that send mail and need updated settings
• File permissions, which should be redesigned rather than copied blindly