With samba package there comes a tool called pdbedit which is used to manage users in the passdb backend. The tool also contains import option which may be used to import the old type of password files in to the new backend.
This option will ease migration from one passdb backend to another.
Example: pdbedit -i smbpasswd:/etc/smbpasswd.old
The only catch is that the import will be incomplete if there is a error in the old password file e.g. there is a user that doesn’t exist in the unix passwd file.