E2K/Domain change

I am changeing our domain name from xxx.com to yyy.com. How do I do this on E2k without loosing any data. I am configuring the new yyy.com on a new box. If I successfuly change that, I want all the email that come in the form of 123@xxx.com to be routed to 123@yyy.com. Please help me how I can accomplish this?…Thank you.