I’ve been doing my own IT for a long while, as well as small tasks for others. But I haven’t had to do an email migration—while ensuring pretty well no downtime—in a long while.
Fortunately the shift from Google Mail (due to the deprecation of grandfathered accounts that offered free custom domain integration) to Apple’s iCloud+ was remarkably smooth and easy. Apple’s instructions were helpful as were those of the host I was dealing with. Downtime was a couple seconds, at most, though there was definitely a brief moment of holding my breath in fear that the transition hadn’t quite taken.