Server 2003 to Server 2012 Domain Controller Migration - IP Address Question

I know, I'm a few months late, but I am finally getting started on my Server 2003 migration project.

I currently have 2 domain controllers running on our domain with Server 2003 and a funtional/forest level of 2003.  I also already have the 2 new Server 2012 domain controllers built out joined to the domain, but I haven't promoted them to domain controllers yet.

My main concern, is that my primary DC is also serving DHCP to clients on our network and it's also the primary internal DNS server as well as NTP.  The new Server 2012 domain controller will also serve these same roles.  All of our current servers DNS address and applications using NTP are pointing to the IP address of our primary DC.  How should I handle this without having to reconfigure every server, desktop, laptop, application, etc on our network?

DC01 - - Primary DC (Server 2003)
DC02 - - Backup DC (Server 2003)

DC03 - - Primary DC (Server 2012)
DC04 - - Backup DC (Server 2012)

All in all, I need DC03 and DC04 to consume the DC01/DC02 IP addresses once the cut-over happens.  Are there certain steps I should go about doing this?  Once I transfer all of the FSMO roles and DC01 and DC02 are demoted, can I simply take them offline and reset the NICs on DC03/DC04 to their addresses?  Any other caveats I should worry about from doing this?

Thanks for all of the help in advance.  This my first DC upgrade, so it will go a long in helping me get the process down.
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Chris HInfrastructure ManagerCommented:
Your problem will be overlap.  Do you have any available room in either of your 10.x.x.x scopes for double growth?  If so, set 03/04 to use unused addresses on DHCP scope and issue new 03/04 DNS server settings on it.
David Johnson, CD, MVPOwnerCommented:
First promote these to a domain controller
export your dhcp settings and add them to a dhcp server on one of the new DC, authorize and add the new DC's addresses to the dns servers list in dhcp
disable the dhcp service on S2K3
setup ntp on the server that will be the PDC
update your gpo to point ntp to the new server.
run in parallel for a while, remove the 2K3 backup dc (dcpromo) , wait and check results,
turn off/unplug network cable 2K3 Primary, wait check results, dcpromo remove 2K3 Primary
now your done.

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Lee W, MVPTechnology and Business Process AdvisorCommented:
Why must they be the same IPs?  This doesn't make much sense if your network was setup properly.
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sciggsAuthor Commented:
All servers and other apps using DNS or NTP are statically assigned the addresses of DC01 and DC02 in their DNS or NTP settings.  My thought was it would keep me from having to change 100+ connection settings to the new IP address.  I understand may have to flushdns but rather keep from changing each connection.  Am I missing something obvious here?
Lee W, MVPTechnology and Business Process AdvisorCommented:
You set NTP via IP and not name (or better still allowed AD to control it)?

And DNS Statically?  Set DNS through DHCP.
Zacharia KurianAdministrator- Data Center & NetworkCommented:
It is not a big deal to change the IP of your windows 2012 Domains. If you follow the comments by @David, you would be on safe side. I have done the same for a couple of customers in windows 2008 R2 & windows 2012. The below links are worth to read too;

sciggsAuthor Commented:
I get what you are saying, they could have been assigned through a DHCP reservation versus manual static, but that's not what we have at the moment.  I'm just looking for the best way to get around having to change these values.
David Johnson, CD, MVPOwnerCommented:
the key is your dhcp settings. clients by default query dhcp and that is where they get their dns and other settings.
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Windows Server 2003

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