Welcome to the forums,
In the admin console gui-simply create a new domain, then open the individual user's settings you would like to change-and simply change the drop down to the other domain)
Don't delete that macserver.domain.com just yet! There's logfiles/daily reports that get sent to
admin@domain.com.
In fact I don't delete it at all:
I move my admin account(s) to the new domain, then in
admin@domain.com I make aliases for
admin@macserver.domain.com,
root@macserver.domain.com,
postmaster@macserver.domain.com,
abuse@macserver.domain.com
Or you could examine your config (zmprov gacf) and change all the references to the admin account, but it's just easier if you leave the extra domain as it was.