This guide will crash course you on migrating DHCP from 2003 to 2012. Basic admin knowledge is advisable.

1. First install DHCP services on Server2012
2. Authorize the new DHCP server on the domain
3. Validate the authorization
   1. On the old server execute this command in prompt:
      netsh dhcp show server
   2. Check that BOTH servers are listed
4. On the old Server 2003 export the current DHCP configuration
   1. netsh dhcp server <\\servername> export <c:\filename> all
5. On the new Server 2012, import the DHCP configuration
   1. Shutdown DHCP service
   2. Delete c:\windows\system32\DHCP\dhcp.mdb
   3. Start the DHCP service
   4. netsh dhcp server <\\servername> import <c:\filename>
      
   5. Restart DHCP service

Now you’re done!

Source: Technet Blogs