The accounting firm serves several dozen companies and runs HR and payroll, so its calendar is brutally predictable. From January to the end of March work runs at full steam: JPK deadlines, CIT and PIT filings, year end closings. In that window every hour of downtime is a real problem, because tax deadlines cannot be moved. It was precisely this seasonality that shaped the whole project.
The starting situation
Email ran on old shared hosting, with no backup and no sensible account protection. Client documents were scattered across local drives and USB sticks, and versions of the same file circulated in email attachments. IT support came down to a call to a friendly IT person who helped when he happened to be free. Nobody could answer a simple question: what happens if someone deletes a case folder or the hosting goes down in February.
Why it was urgent
In an accounting firm, losing correspondence with a client or a source document is not just a technical nuisance but a risk of missing a deadline and being liable to the client. The old hosting gave no guarantee of data recovery, and the lack of a backup meant that a single human error could delete work with no way to undo it. The owner knew the change had to happen, but not in the middle of the season and not at the cost of even a single day of downtime.
How we ran the project
We started with the schedule, not the technology. We agreed to complete the whole Microsoft 365 migration outside the peak, with the actual email cutover over the weekend. For several days before the switch we synchronised the mailboxes in the background, so that when the MX records changed most of the data was already in place and only a short final sync remained.
We moved the message history, contacts and calendars to Exchange Online in full. On the file side we did not simply move the mess to a new place: we organised the structure in SharePoint by client and document type, and current staff files went to OneDrive. After the migration we compared the message count in each mailbox with the starting state, to be sure nothing had gone missing.
In parallel we deployed an independent Microsoft 365 backup covering email, OneDrive and SharePoint. Copies go to a location independent of Microsoft, with 12 months of retention matched to the way accounting documents are stored. We check recoverability daily, and we ran a test restore of a single file and of a whole mailbox together with the team, so they knew how it looks in practice.
Finally we put support in order. Instead of ad hoc calls the firm got one ticketing channel, a helpdesk with a guaranteed response time and environment monitoring. That is the most important change for the season: when something stops working in February, it is clear who to call and how quickly the problem will be picked up.
What changed
The email cutover fit within the weekend window and on Monday the team was already working in Microsoft 365, with the full correspondence history. Not a single lost message was reported. Documents stopped circulating in attachments: people work on one, current version in SharePoint, and a deleted file comes back in minutes from the backup, without asking anyone for a favour.
The firm entered the next filing season with a clear picture of its IT: data copies are tested, accounts are secured, and ongoing care provides a predictable response time instead of uncertainty. The change the owner feared as a risk turned out to be what lifted the most daily stress from her shoulders.