Lex, I think you have to take it out of the domain, join it to another, then rejoin it, then reboot.
EDIT: Ahh, "contoso" brings back memories...That is the example in the books I used in classes - Microsoft series. What we did in class to was to use Virtual Server 2005 and like Osiris said, we used virtual machines - one domain for contosoxx and one domain for contosoyy, under the parent domain, contoso. It was easy to learn that way, because you could see how the two virtual domains interact, plus we also used Outlook Web Access in tandem with Exchange, to learn more about SMTP messages, mailboxes, etc . Might be a good way to learn, worked well I have to say.