How to Migrate to Microsoft 365 Without the Downtime Headache

Why the migration itself is the risky part

Most small businesses that move to Microsoft 365 are happy they did. The trouble is that the move is a one-off job, so nobody in the office has done it before, and the mistakes tend to be the same ones every time: mailboxes deleted too early, a DNS change made at the wrong moment, or a shiny new system left wide open because everyone was relieved to be finished.

This guide walks you through the move the way we would run it for a client — in plain English, with the security work treated as part of the job rather than an afterthought. If you would rather not touch DNS records yourself, that is a perfectly sensible decision, and a good managed provider will do the technical parts while you sign off the plan. If you are weighing that up, our overview of what managed IT support actually covers explains where the line usually sits.

Step one: take stock before you touch anything

A good migration is mostly preparation. Before you sign up for a single licence, write down what you actually have:

• Every mailbox and shared mailbox — names, addresses, and rough size. A ten-year-old mailbox can be surprisingly large, and size drives how long the copy takes.
• Where your email lives now — an old hosting provider, a previous IT company, or a self-managed server. You will need the login to make changes.
• Your domain registrar — the company you bought yourbusiness.co.uk from. This is where DNS records are changed, and it is astonishing how often nobody in the business remembers the login.
• Shared files and folders — network drives, a file server, Dropbox, whatever you use. Note roughly how much data there is and who needs access to what.
• Anything that sends email as you — your website contact form, an accounting package, a booking system. These often have their own settings that need updating.

This inventory is the single most useful thing you can produce. Half of all migration surprises are simply things nobody wrote down.

Step two: pick the right plan, not the biggest one

Microsoft sells several Business plans, and the differences matter for a small team. The cheaper plans give you email and the web versions of the Office apps; the mid-tier plans add the installed desktop apps; the Premium plan adds the security and device-management tools that many small businesses genuinely benefit from.

Don’t overbuy, but don’t cripple yourself to save a few pounds a month either. If security and compliance matter to your trade — and for accountants and solicitors they absolutely do — the extra controls in the higher plans usually pay for themselves the first time they stop something. You can mix plans across staff, so directors who need the full toolkit can be on a richer plan than occasional users.

Step three: set up the new home before you move in

Create your Microsoft 365 tenant, verify that you own your domain (Microsoft gives you a code to add to your DNS records — this proves ownership without changing where mail is delivered yet), and create the user accounts. At this stage nothing is live: you are building the new house while everyone still lives in the old one.

This is also the moment to plan the data copy. Mailboxes are migrated using a tool that copies everything — emails, calendars, contacts — into the new accounts in the background while the old system keeps running. Files can be moved into OneDrive and SharePoint. The key principle is: copy first, cut over second, delete last.

Step four: the cutover — changing DNS and MX

This is the part everyone worries about, and it is genuinely the only moment staff will notice anything.

Your MX record is the signpost that tells the rest of the internet where to deliver your email. Right now it points at your old host. When the mailbox copy is complete and verified, you change the MX record to point at Microsoft 365, and update the related records Microsoft asks for (SPF, and ideally DKIM and DMARC, which help stop criminals forging email from your domain — worth doing properly given how common business email compromise has become).

A few things make this go smoothly:

• Do it at a quiet time. A Friday evening or a weekend means fewer emails in flight during the switch.
• Expect a few hours of overlap. DNS changes take time to spread across the internet, so for a short window some mail arrives at the old host and some at the new one. This is normal.
• Keep the old system running for a week or two. Do not cancel or delete anything the moment cutover finishes. The old mailboxes are your safety net until you are certain nothing was missed.

Once the new MX record has spread, all mail flows into Microsoft 365, staff sign in with their new accounts, and the old system quietly goes idle.

Step five: lock it down — the step most people skip

Here is the honest bit. A brand-new Microsoft 365 tenant is not as secure as it should be out of the box, and the day the migration finishes is exactly when everyone stops paying attention. Do not let that happen.

At a minimum, before you call the job done:

• Turn on multi-factor authentication for everyone. This is the single biggest thing you can do to stop an account being taken over, and Cyber Essentials already requires MFA on cloud services. Our guide to multi-factor authentication explains it without the jargon.
• Switch off legacy sign-in methods that MFA cannot protect. Criminals target these deliberately.
• Set sensible sharing rules so files are not accidentally shared with the whole internet.
• Protect the administrator account with a strong, unique password and its own MFA — it is the master key to everything.

We have written a full, plain-English checklist of the Microsoft 365 security settings worth enabling, and we would genuinely urge you to work through it before you move on. A migration that ends with an unprotected tenant has swapped one problem for a bigger one.

Step six: confirm, then clean up

Give it a fortnight. During that time, check that:

• Mail is arriving and sending normally for everyone.
• Calendars, contacts and shared mailboxes came across intact.
• Your website form, accounting software and anything else that sends email as you is now pointing at the right place.
• Files are where staff expect them and permissions are correct.

Only when all of that is confirmed should you decommission the old system. Keep a final backup or export of the old mailboxes somewhere safe before you close the account — it costs almost nothing and it is the difference between a minor and a major bad day if something turns up missing later.

Do you need to do this yourself?

A confident owner with a small team and simple needs can run a Microsoft 365 migration over a weekend. But the DNS cutover, the security hardening, and the “copy first, delete last” discipline are exactly the places where a steady hand saves you from a bad week. There is no shame in handing the technical work to someone who does it regularly while you keep the business running.

If you would like the move planned and run properly — with the security done as part of the job, not bolted on afterwards — take a look at our managed IT services or just get in touch for a straight conversation about what your setup needs.