Right, SOGo will use the new mail address but you still need a mail alias in server manager and the default address can’t be deleted in server manager but made internal instead.
For AD you need to edit the users userprincipalname field.
I recommend to use phpldapadmin:
https://wiki.nethserver.org/doku.php?id=phpldapadmin
For command line editing samba AD check out the wiki: (scroll to “Modify the SAMBA4 AD settings”)
https://wiki.nethserver.org/doku.php?id=howto:useful_commands#samba4