Remove Active Directory from 2003 Server R2 after migrating to 2012 R2 Essentials

I replaced an old Windows 2003 R2 server with 2012 R2 Essentials as per this article
FSMO roles successfully transferred, and all users happily accessing their files on the new server.
The only thing left is to remove AD from the old server.
dcpromo fails with the following error:

The operation failed because: Managing the network session with SERVER16.aaa.local failed
"The network path was not found. If this computer is connected to the network via a Remote Access Service (RAS) connection, ensure that File and Printer Sharing for Microsoft Networks is enabled for that connection."

Another symptom is that the 2003 R2 server can't access the shares on the 2012 R2 server.
This is to be expected, since 2003 is not supported anymore.

What would be the best way remove the old server from AD?
1. Temporarily downgrade security on the 2012 serve to allow connection from 2003
    (enable SMB1 etc.) Not sure if this would still work at all.
2. Use one of the AD cleanup methods to remove the old server from AD
    Which method would be best? Can this be done using PowerShell?