Trying to promote 2012 Essentials R2 Domain Controller on SBS 2003 network

First of all there are limits to domain controllers in SBS domains, the SBS domain controller must hold the FSMO roles
http://blogs.technet.com/b/sbs/archive/2007/10/04/debunking-the-myth-about-additional-domain-controllers-replica-dcs-in-an-sbs-domain.aspx
However the same limitation applies to Server 2012 Essentials

Windows Server Essentials Experience in Windows Server 2012 R2 Essentials      
Must be the domain controller at the root of the forest and domain, and must hold all the FSMO roles.
Cannot be installed in an environment with a pre-existing Active Directory domain (however, there is a grace period of 21 days for performing migrations).

This refers to the special migration mode install of 2012 essentials
http://technet.microsoft.com/en-us/library/jj200112.aspx

If this isn't working, another server is needed.  Your domain migration has become much more complex, requiring another windows domain controller onto which you seize the fsmo roles while decommissioning the SBS 2003 controller, followed by another seizure to complete the migration.  Painful.

This versions are not like regular versions to work with dcpromo.

You must first, make sure you did follow the wizard (2012) by selecting the installation mode: Server migration.

If you did select that option then you must complete all step to make sure your server is in control, like joining the computers to the new server, moving any data, exchange (if any), transfer GPOs, DHCP, DNS, scripts, printer, etc.

Then you can run dcpromo on the 2003 to remove the AD DS role to make it a domain member server, please Do not select "This server is the last domain controller in the domain." from the options. By demoting the 2003 will automatically make your 2012 the main DC.  You may shutdown both servers and restart your 2012 first and your 2003 after just to confirm it is in fact just a member;  Connect some PCs and troubleshoot if required.