How do I go about transitioning to a new email domain name?

Here is my scenario:

Currently, we have a domain, let's call it XXXX.  All of our email addresses however, are firstname_lastname@YYYY.org.  We'd like to transition our email addresses to @ZZZ.org.  The domain will stay the same (XXXX), just the email suffix will change.  However, we'd also like to keep @YYYY.org active for the external world who will obviously still be sending to @YYYY.org.

Is this just adding a new entry into the default recipient policy in Exchange?  Making @ZZZZ.org the primary, but still keeping @YYYY.org checked?  Any advice would be much appreciated!