Dave:
Little known fact: the Administration server for the domain is the admin. server for the Domino Directory (Address book, or NAB).
First, change the administration server of the NAB by going into the ACL of the NAB, click on the Advanced tab and select the server you want to be admin. server. It may ask you if you want to delete the old server's entry in the NAB: say "no".
Second: replicate this change to all servers in the domain.
From there, it's a matter of cleaning up places where you don't want the old server to be in person records, etc.
HTH.