Hello,

When you’re evaluating Office 365, you usually create “Cloud Only” account. When you start your migration, you want to match your Online users with your on-premise user with Azure Active Directory Sync (AADSync) to avoid dataloss. You have two options : Soft match, and hard match.

Soft Match :

You match the two users with a unique User Principal Name, the Online users become a Synchronized users.

Hard match :

You force the ImmutableID of the Online user to be a calculation of the ObjectGUID of the On premise user :

[Guid]$UserGuid = Get-ADUser -Identity SamAccountName| Select-Object -ExpandProperty ObjectGuid
$bytearray = $UserGuid.ToByteArray()
$immutableID = [system.convert]::ToBase64String($bytearray)

Set-MsolUser -UserPrincipalName OnlineUPN -ImmutableId $immutableID