In order to transfer AD from old to new, the servers will have to be live on the network for the duration of the transition time. This being the case, the new server needs to have a different name and IP address than the old server.
They won't clash, and Active Directory was designed to be able to run on multiple servers on the network for reasons of redundancy and for promoting/demoting servers (reference DCPROMO).
Suggest reading up on Active Directory and DNS as it pertains to AD. ALso, what other services is the older server providing (File, Print, DHCP, etc.)?? You'll need to make a plan for maintaining or transitioning these as well.
High level view:
- bring up new server on network with new name and static LAN IP
- configure remainder of network settings on new server (particularly DNS)
- JOIN the new server to the AD
- DCPROMO new server as a new DC in the existing AD organization
Don't know what you're doing with the old server, i.e. if you're replacing the old with the new, or keeping the old server on board for AD redundancy. The answer to this determines what you'll do relative to
- Global catalog
- FSMO roles
- DHCP
- "Other" services, like file/print sharing from old DC, etc.
Transferring AD is the easy part, it's the other services that the current DC may be providing that require some care and finess.