Just tested inplace upgrade of Windows 10 Core/Home to Windows 10 Education dism /online /Set-TargetEdition command failed. Did Slui method. Input Windows 10 Education KMS Ckient Key XXXX-XXXX-XXXX-XXXX-VCFB2 Upgrade went fine. Reboot and system is now Education Edition. MS mentions it equals to Windows 10 Enterprise
I've tested both: applying offline upgraded WIM & upgrade online. For both cases, "Windows 10 Education" watermark remains on the desktop. Toggling the value of "PaintDesktopVersion" from HKEY_CURRENT_USER\Control Panel\Desktop doesn't affect anything. Seems that the watermark isn't dependent on activation status too.
In VM one could check to remove with a WM Remover and see whether the WMIC OS Name branding is crippled . Like it did with the Eval Watermarks. wmic os get caption should state 'Microsoft Windows 10' in case.
I have a 5 year old Dell Latitude laptop I use for testing. I did an in-place upgrade from Windows 7 Home Premium (activated) to Windows 10 Home (activated). I entered the education key using 'slui'. It did the upgrade. There's a Windows 10 Education watermark. But now it's no longer activated. I'll restore Windows 10 Home from a full backup. I wish this had worked - and remained activated. Maybe someone will figure out a way to do it and retain the activation.