I'd just do a simple DCPROMO to make it a domain controller then migrate any FSMO Roles that the old server holds and then demote the other server back to being a normal member server. Add it as a secondary DHCP and DNS Server and then remove the services from the old one. finally i'd replicate the data (shares) I assume and then over night I'd do a final update of the new shares before powering off the old server and putting in a DNS alias pointing to the new server.