A 340-seat manufacturer, twelve years of on-prem Exchange, and a hard cutover window of 58 hours. Here is what we would tell anyone standing where we stood on that Friday afternoon.
Plan for the data you forgot about
Mailbox count is the easy number. The project was sized by mailbox count and nearly blown up by everything attached to it: 41 shared mailboxes, six years of PST files scattered across desktops, public folders nobody had opened since 2019, and one distribution list that turned out to be load-bearing for the entire shipping department.
The weekend, in order
friday 18:00
Final delta sync started, mail flow still pointing at on-prem so nothing was in flight when we moved.
saturday 04:00
MX cutover. This is the only truly irreversible step of the weekend; everything before it is rehearsal.
saturday 09:00
Desktop profiles re-provisioned in rings of 40. Autodiscover handled most of it; 22 machines needed hands on keyboard.
sunday 14:00
Public folders landed last, deliberately. Nobody was blocked waiting on them, so they got the slack in the schedule.
What we would do differently
- Inventory PSTs two weeks earlier. They were the only workstream that ran long.
- Pre-stage the shared-mailbox permission map as code. We rebuilt it by hand and it cost four hours we did not have.
- Publish a one-page "what changes Monday" note to staff on Thursday, not Sunday night.
Migration-related tickets on Monday morning. For a cutover, that is the only score that counts.