As you may know, Active Directory provides authentication and authorization mechanisms as well as framework from within other related services that can be deployed. As an LDAP compliant database, it contains most commonly used objects such as users, computers, and groups which can be organized into organizational units or OUs by any number of logical or business needs. Group Policy Objects or GPOs are then linked to OUs to centralize the settings for various users or computers across an organization. Part of the quandaries that IT professionals face is taking advantage of nuances provided in Active Directory in newer server offerings such as Windows Server 2012.  As detailed in Pierre's post, "Windows Server 2012 Active Directory – What’s New?", Active Directory provided in Windows Server 2012 is provided impactful enhancements.  Yet some organizations choose not to migrate due to reasons of uncertainty.
This Step-By-Step, suggestion online via the IT Professionals Community of Greater Toronto, is to help deal with that uncertainty and provide guidance for IT professionals looking to migrate their organizations Active Directory offering from Windows Server 2003 to 2012.

Prerequisites
  1. Download Windows Server 2012. If you plan on completing this Step-By-Step in a virtual lab, it is recommended to download the FREE Hyper-V Server 2012 first.
  2. Complete Step-By-Step: Adding a Windows Server 2012 Domain Controller to an Existing Windows Server 2003 network

Transferring the Flexible Single Master Operations (FSMO) Role
  1. Open the Active Directory Users and Computers console on your new Windows Server 2012 computer.
  2. Right click your domain and select Operations Masters in the sub menu.
  3. In the Operations Masters window, ensure the RID tab is selected.
  4. Select the Change button.

  5. Select Yes when asked about transferring the operations master role.
  6. Once the operations master role has successfully transferred, click OK to continue.
  7. Ensure the Operations Master box now shows your new 2012 Windows Server.
  8. Repeat steps 4 to 6 for the PDC and Infrastructure tabs.
  9. Once completed, click Close to close the Operations Masters window.
  10. Close the Active Directory Users and Computers window.

Changing the Active Directory Domain Controller 
  1. Open the Active Directory Domains and Trusts console on your new Windows Server 2012 computer.
  2. Right click your domain and select Change Active Directory Domain Controller... in the sub menu.
  3. In the Change Directory Server window, select This Domain Controller or AD LDS instance.
  4. Select your new 2012 Windows Server.

  5. Click OK to continue.
  6. Back in the Active Directory Domains and Trusts window, hover over the Active Directory Domains and Trusts found in the folder tree on the left hand side to ensure the server now reflects your new 2012 Windows server.
  7. Right click Active Directory Domains and Trusts found in the folder tree and select Operations Manager... in the sub menu.
  8. In the Operations Master window, click Change to transfer the domain naming master role to the 2012 Windows Server.
  9. When asked if you are sure you wish to transfer the operations master role to a different computer,click Yes.
  10. Once the operations master is successfully transferred, click OK to continue.
  11. Click Close to close the Operations Master window.
  12. Close the Active Directory Domains and Trusts console.

Changing the Schema Master
  1. Open a command prompt in administration view on your new Windows Server 2012 computer.
  2. On the command prompt window, enter regsvr32 schmmgmt.dll and hit enter.
  3. Once completed successfully, click OK to close the RegSvr32 window.

  4. Close the command prompt.

Add the Active Directory Schema Console from MMC
  1. Open a MMC console on your new Windows Server 2012 computer.
  2. Click File > Add/Remove Snap-in...
  3. In the Add or Remove Snap-ins window, select Active Directory Schema and click the Add >button.

  4. Click OK to continue.

Change the Schema Master
  1. In the same MMC console, right click Active Directory Schema and select Change Active Directory Domain Controller... in the sub menu.
  2. In the Change Directory Server window, select This Domain Controller or AD LDS instance.
  3. Select your new 2012 Windows Server.
  4. Click OK to continue.
  5. A warning will appear stating that the Active Directory Schema snap-in in not connected. Click OK to continue.
  6. Hover over the Active Directory Schema folder in the folder tree to ensure the new Windows Server 2012 computer is shown. 
  7. Now right click Active Directory Schema and select Operations Master... in the sub menu.
  8. In the Change Schema Master window, click Change to transfer the schema master role to the 2012 Windows Server. 
  9. When asked if you are sure you wish to transfer the schema master role to a different computer, clickYes.
  10. Once the schema master is successfully transferred, click OK to continue.
  11. Click Close to close the Change Schema Master window.
  12. In the MMC, click File > Exit.
  13. When asked to save the console, click No.
 Once completed, open the Active Directory Users and Computers console to verify that the Active Directory database successfully replicated to your new Windows Server 2012 computer.  Be aware that the database replication may take some time depending on the number of objects in Active Directory.

Removing the 2003 Windows Server from the Global Catalog Server
  1. Open Active Directory Sites and Services on your new Windows Server 2012 computer.
  2. Expand the Sites folder, then the Default-First-Site-Name folder, then the Servers folder.
  3. Expand both listed servers. One should be your new 2012 Windows Server and one should be you 2003 Windows Server.
  4. Right click NTDS Settings found under your old 2003 Windows Server.
  5. In the sub menu, select Properties.
  6. Under the General Tab, unselect Global Catalog and then click the Apply button.
  7. Click OK to continue.
  8. Close the Active Directory Sites and Services window.
  9. Verify that your new 2012 Windows Server is running the FSMO role by opening the command prompt in Administrative view and running the following command: Netdom query fsmo.
  10. In the Network and Sharing Center, be sure to change the Preferred DNS server to match the Alternate DNS server, then delete the IP address listed under the Alternate DNS server should it currently be pointed to the old 2003 Windows Server.

All that's left is to demote the old 2003 Windows server by first adding the new 2012 Windows Server as the Primary DNS, followed by running DCPROMO to demote the old 2003 Windows server.