Sorry Denis, you missed the step-by-step process by not using the migration white paper procedures. There is no basic after the fact easy step-by-step process. What about the company website? User data? Fax? Printers? How do your two SBS servers like fighting over DHCP?
When you say, move the clients to the new server, are you referring to the users or computers or both? Were you expecting to use the SBS2008 connect computer wizard? If so, have you tried that on a workstation that was previously a member of the SBS 2003 domain?
IMHO, You were given very bad advice.