Why don’t you add the server 2008 servers as DC and demote the old servers. You can assign the new 2008 server with different names and ip. What you need to change is your dns configure on the DHCP servers to point to these new servers for DNS services. Hope that helps.
Since you have 4 DCs and a primary and secondary DNS, you can take one 2003 DC down and replace it with a 2008 DC without anybody noticing any difference. Here are the steps:
1. Prep the 2003 forest and domain for server 2008.
2. Setup new a new Server 2008 server running a fresh OS (unmodified).
3. Select a 2003 DC to demote and move all master roles and external LDAP configurations to one of your other servers. (This can even be your primary DNS server)
4. Demote the DC then remove it from the domain with a different computer name. (make sure you know the local admin password)
5. Login again and clear the IP address.
6. Shutdown the old DC.
7. On the Server 2008 machine, set the IP address and add it to the domain.
8. Promote the Server 2008 server to a DC.
9. Check for successful AD and DNS replication.
10. Move on to the next server.
Step 3 is very important in making this a smooth transition. If an application like a spam appliance uses LDAP, you need to make sure you don’t forget to change the LDAP server it is referencing before taking that server down. If you have a single DHCP server, you should make a secondary DHCP server at least during the transition.