Hello,
My name is Jobin Johny. Am a senior Microsoft system administrator. I can help you in this task.
Here is the High level points, and i will share the full design and details when you award the job.
1. Deploy the Windows 2012 server
2. Add it to the Existing Forest-Domain
3. promote the server as Domain controller
4. Seize the FSMO Active Directory Roles
5. Decommission the 2003 server ,
6. Remove from Network
You can contact me on skype also, my id : eynetech