Samba-3.0.0 default behavior should be approximately the same as Samba-2.2.x. The default behavior when the new parameter passdb backend is not defined in the smb.conf file provides the same default behviour as Samba-2.2.x with encrypt passwords = Yes, and will use the smbpasswd database.
So why say that behavior should be approximately the same as Samba-2.2.x? Because Samba-3.0.0 can negotiate new protocols, such as support for native Unicode, that may result in differing protocol code paths being taken. The new behavior under such circumstances is not exactly the same as the old one. The good news is that the domain and machine SIDs will be preserved across the upgrade.
If the Samba-2.2.x system was using an LDAP backend, and there is no time to update the LDAP database, then make sure that passdb backend = ldapsam_compat is specified in the smb.conf file. For the rest, behavior should remain more or less the same. At a later date, when there is time to implement a new Samba-3 compatible LDAP backend, it is possible to migrate the old LDAP database to the new one through use of the pdbedit . See Section 10.3.2.