Hi,
To keep the same IPs, here's basic steps you can follow.
1. Setup the 2012 R2 servers as DCs and DNS with different IPs on the same subnet.
2. Shutdown one of your two 2003 R2 DC/DNS servers (service should still be available due to the secondary DC/DNS server that is still up).
3. Change IP of one of your two 2012 R2 servers to the IP of the server you just shut down.
4. Either reboot the 2012 R2 server or, ipconfig /registerdns and restart the netlogon service.
5. Disconnect shutdown 2003 R2 server from network, change IP and reboot or /registerdns and restart netlogon. (Might as well reboot and reconnect since there should be any service impact.)
6. Transfer FSMO roles to the 2012 R2 server, give some time to replicate.
7. Demote the 2003 R2 domain controller you've changed.
Repeat steps for the second 2003 R2 and 2012 R2 servers (transferring of FSMO roles will be optional).
Some articles for your reference:
Step-By-Step: Active Directory Migration from Windows Server 2003 to Windows
Server 2012 R2
https://blogs.technet.microsoft.com/canitpro/2014/04/01/step-by-step-active-directory-migration-from-windows-server-2003-to-windows-server-2012-r2/
Upgrading an Active Directory Domain from Windows Server 2003 or Windows Server 2003 R2 to Windows Server 2012 or Windows Server 2012 R2
http://blogs.msmvps.com/mweber/2012/07/30/upgrading-an-active-directory-domain-from-windows-server-2003-or-windows-server-2003-r2-to-windows-server-2012/
Best Regards,
Alvin Wang
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