As part of your preparation of your existing domain, you need to change the IP address of the Secondary DNS server on your existing Domain Controller to be the IP address of your new Domain Controller.
You can find out the IP address of your new Domain Controller from the Commissioning Console.
Step 1. Log on locally to the domain controller whose IP address you want to change.
Step 2. Click Start, point to Administrative Tools, click Server Manager, and then click View Network Connections.
Step 3. In the Network Connections dialog box, right-click the appropriate connection, and then click Properties.
Step 4. In the Connection Properties dialog box, double-click Internet Protocol Version 4 (TCP/IPv4)
Step 5. In Alternate DNS server, type the IP address of your new Domain Controller (you can see this IP address in the Commissioning Console or the Welcome email)
Step 6. Click OK to close the Internet Protocol (TCP/IP) Properties dialog box.