Initially, I tried to bring a Windows 2012 R2 into the 2003 ADS environment directly. In other words, I installed 2012 server and tried to promote it in an environment with 2003 DCs exclusively. This failed miserably and instead of spending a lot of time trying to figure the issue out, I opted to go to 2008 R2 and then 2012 R2. This scenario went much smoother.

This was a single Windows 2003 domain. Very simple environment. All role installation and promotion was done from the domain Administrator account.

I built a Windows 2008 R2 server and joined it to the domain.

Then, I added the Active Directory Domain Services role, and let the wizard install the DNS Server role as well.

Once the roles were installed and server rebooted, if needed, I verified the time and that DNS was set correctly.

Then, I ran dcpromo.

When dcpromo was finished, I rebooted and logged in the domain Administrator account.

Then, I transferred all the FSMO roles to the new Windows 2008 R2 domain controller (see ).

I built and joined a Windows 2012 R2 server to the domain.

I added the Active Directory Domain Service role, and again let the wizard install the DNS Server role.

Once the roles were installed, and I verified the DNS and time, I ran the dcpromo equivalent in Windows 2012 R2. I clicked on the flag with the warning symbol in the top right of the Server Manager window. In the drop down, under the Active Directory Domain Services role, was a link to promote the server. I clicked on it to start the promotion. Note: dcpromo is not supported in 2012. When you try to run it, you get a message telling you to go to Server Manager.
dcpromo equiv from server manager make sure to select the 2008 server to replicate from.

After the server rebooted, I transferred all the roles, in a similar manner as above, to my Windows 2012 R2 server.

Once that was done, I verified replication and authenticated to the 2012 server from a client.

Then, I shutdown my 2003 domain controller.

In a week or maybe two, I will boot the 2003 server and demote it. I don’t want to demote right away in case something goes wrong or comes up.
demote 2003 box or shutdown for a while.

Then, I will demote the 2008 server as well. If there were more than one domain controller in this environment, I would keep the 2008 server up until all the domain controllers were replaced with upgraded ones.

UPDATE: So, I demoted the 2003 server after checking to make sure all the roles were on the 2012 server, and ensuring that the Global Catalog was on the 2012 server too. To demote, I just ran dcpromo.

Then, a couple days later, I went through the same process on my 2008 server. I ended with only one domain controller which is a Windows 2012 R2 server.