What version of Samba the old server is? And what type of a domain? Windows NT domain or an Active Directory domain? If it is the old one, which I suppose, because there was no support for being the AD domain primary controller in the 3.x Samba versions, that probably something like adding the new server to the domain as secondary/backup controller, syncing domain, promoting new server to primary domain controller and removing the old one may work. Consider this only as guidance for better keywords for Google, because it was many years ago when I was fluent in the Windows tribe language