This information just came in today:
Windows 10 Client Connector for connecting to Windows Server 2012 R2 is now Released via Download Center
We have now officially released the Windows 10 connector for connecting to Windows Server 2012 R2 via Download Center. It is available from the following URL:
Please note: we do not have the fix for the auto-download and installation released yet. So you have to manually download and install this client connector on the Windows 10 client machine. Then you can run the client deploy to start the connecting.
For the servers in-place upgraded from Windows 7, 8.1 to 10, you will need to manually download and install this client connector and do the connecting again.
If the machine ID is the same after the upgrade, then everything will be working automatically. However if the machine ID is somehow changed during the upgrade, then it will be treated as a new client, so you have to leave the domain and connect to the Essentials server again. The old client back up data will be in the archive folder on the Essentials server, and you can still use that to do the restore back to the upgraded Windows 10 client.