Log on to old DHCP server, open administrative command prompt and type
netsh dhcp server export \\newserver\c$\dhcp.txt all
and stop and disable the dhcp server service on old server.
Then log on to new server, install DHCP service and authorize it
Then open administrative command prompt on new server and type
netsh dhcp server import \\newserver\c$\dhcp.txt all
or just
netsh dhcp server import c:\dhcp.txt all
If you are using DHCP relay in firewalls, then you might need to reconfigure them also to point to new server.
