I have successfully registered, then upgraded Windows 8.1 Pro and Windows 7 Pro workstations to Windows 10 Pro in a workgroup environment, but can't run the registration wizard on Windows 8.1 Pro workstations that are in a Windows Server 2012 R2 Essentials domain.  Is there some other way to upgrade a Windows 8.1 Pro workstation that's in a domain to Windows 10 Pro?

Hi,

Please try MDT to upgrade to Windows 10.

Upgrade to Windows 10 with the Microsoft Deployment Toolkit
https://technet.microsoft.com/library/mt426643.aspx