Applicable from Windows 2008 up to 2012 R2
When you want to migrate DHCP from one server to another, and you want to keep the DHCP scope as is; you will always have the challenge with current DHCP Leases that already are given to clients.
To overcome this; we can easily migrate the leases from the older server to the new one.
Firstly prepare your NEW server with the DHCP feature; also be sure that this server is 2012 or up and it's ready to serve it's role.
After that you must run this in Power Shell; I always prefer Power Shell ISE for this, because you can see the code an easily adjust where needed. Also don't forget to run Power Shell in Admin Mode.
Export-DhcpServer –ComputerName oldserver.domain.extension -Leases -File %userprofile%\Desktop\dhcpexp.xml -verbose Import-Dhcpserver –ComputerName newserver.domain.extension -Leases -File %userprofile%\Desktop\dhcpexp.xml -BackupPath C:\windows\temp\ -verbose
After that, just disable the DHCP feature on your old server. Also be sure that any IP helpers (if applicable) are updated to point to your new server.