change domain name

[AbsyntH]

Hi
I've a zentyal server in my local network, and i've my mailbox on my ISP (domain mail.it), i've configured the office module with the mail.it domain/realm/openchange and the same for mail module(i'll use fetchmail)...as i can see this is not a good idea, so i wont change my domain settings, i've added mail.local in the dns module,now how i can change mail.it to mail.local in the domain/realm/openchange without reinstall all zentyal server?

[AbsyntH]

of course i wont to send also the email using mail.it domain