The MSOnline Powershell way (deprecated)

You’ll need the object Guid of the object you want to disconnect and set it’s source anchor to $null.

Connect-MsolService
Set-MsolUser -ObjectId <GUID> -ImmutableId $null

After deleting the source anchor, run a sync and you should be good.

Start-AdSyncSyncCycle -PolicyType Delta

The new MsGraph Powershell way

Let me find out…