Two jobs in September came down to the same step. A community kindergarten was moving off Google Workspace, and a financial planner was moving his email from a web host to Microsoft 365. In both cases the day the email actually switches over is called the cutover, and once you know how it runs, there isn't much to it.
What a cutover is
Your email address ends in your domain name. At the company your domain is registered with (the registrar), there are DNS records. They're a set of settings that tell the rest of the internet which system receives mail for your domain.
The cutover is when we change those records to point at Microsoft instead of the old system. Any mail sent after that lands in your new Microsoft 365 mailbox. Because the change happens at the registrar, we need access there before anything else.
Why a Friday afternoon
I start the cutover around 3 or 4pm on a Friday. From then on, staff stop sending email.
Most of the time the new records are picked up and mail is flowing again by early evening. Officially, though, a DNS change can take 24 to 48 hours to reach everywhere, so a bit of mail can still trickle into the old system for a while.
The weekend covers that. The change gets time to settle while hardly anyone's emailing, and if something doesn't look right, there's room to roll back to the old setup before Monday.
On one of these jobs we picked the Friday a week later instead of rushing the one coming up. The cutover only goes smoothly if everything else is ready by that afternoon, so I'd always take the extra week.
Access to the registrar
Sort this out first. You need to be able to log in wherever the domain is registered, or the person who can needs to make the change.
On one of these jobs, the registrar wouldn't talk to me until the client gave them a written authorisation. It was a single line along the lines of "I authorise Brayden from Avera Co to contact you on our behalf", sent from the client to the registrar. Easy once you know it's needed.
If you don't know where your domain is registered, find that out before you book a cutover.
Monday morning
On Monday everyone signs in with their fresh Microsoft 365 login, and anything sent over the weekend is sitting in the new inbox. We run a session that morning to get each person signed in on every device they use: the work computer, the phone, the laptop at home.
Files can be simpler
Email is the bit that has to move all at once. Files usually don't.
The planner's personal OneDrive had business files mixed in with plenty that had nothing to do with the business, things like kids' sport and personal tax. There was more to leave behind than to bring across, so we dragged and dropped the business files into his new business OneDrive instead of running a full migration.
Our guide to choosing a Microsoft 365 plan covers the plan side, and the move itself is part of our Microsoft 365 setup. If you're thinking about a move, the easiest first step is to check you can log in to your domain registrar.