Merge ADDS user with existing Azure AD user.

Start-ADDSAADUserMerge.ps1

# Get the Immutable ID of the ADDS user
$Name = 'My Name'
$ADUser = Get-ADUser -Filter 'Name -like "*$Name*"'
$ImmutableId = [System.Convert]::ToBase64String($ADUser.ObjectGUID.tobytearray())

# Connect to Azure AD
Connect-AzureAD

# Remove the duplicated user from Azure AD that was created when the ADDS user was first synced
Remove-AzureADUser -ObjectId <duplicated AAD user ObjectId>

# Set Immutable ID of existing AAD user to the value obtained from the ADDS user
Set-AzureADUser -ObjectId <existing AAD user ObjectId> -ImmutableId $ImmutableId

# Start AAD Sync cycle
Start-ADSyncSyncCycle -PolicyType Delta