Restarting Directory Servcices from the Command Line: OS X

killall -HUP DirectoryService

DHCP will not authorize after a re-IP of the server

You must first unauthorize a DHCP server scope prior to re-IPing the server.  If you do not then you will be unable to re-authorize the scope after the re-IP.

If you have already re-IPed the server without first authorizing the scope open DHCP from Admin Tools, right-click on "DHCP" and go to "Manage Authorized Servers."  Find the old DHCP server name/IP and click on "Unauthorize."

Once you have removed the old DHCP server you can authorize the re-IPed server.