You have a 2003 source DHCP server and want to migrate DHCP over to your new target Windows Server 2012.
There are a few methods to achieve this, however i like the simplest and definitely the fastest method, which is pretty much identical to a Microsoft KB article which i have cited below.
I am aware of Server Migration Tools in server 2008 and 2012, however for a quick migration of DHCP the method i use below works best for me.
Firstly from your 2003 server, open a command prompt and type;
netsh dhcp server export c:\dhcp.txt all
Now move this file (dhcp.txt) over to your new server 2008/12 machine, the C:\ drive is fine.
RDP/Connect to your new server, first stop (if started) the DHCP server service. Navigate into C:\Windows\System32\Dhcp and rename/delete dhcp.mdb. In some cases you will need to give yourself explicit modify or higher rights to the file to be able to delete it. Once deleted you can now start the service back up. Note this will recreate a new dhcp.mdb file. Now open a command prompt windows and run this command to import your DHCP configuration into this new database.
netsh dhcp server import c:\dhcp.txt
You should see a success notification. If successful, start up DHCP manager and verify that your configuration has copied over, looks for scopes, dhcp options and dhcp leases (ip addresses).