cd /usr/local/directadmin/scripts
./getLicense.sh 1597 ZZZZ
service directadmin restart

cd /usr/local/directadmin/scripts  
./ipswap.sh 111.111.111.111 222.222.222.222

/etc/init.d/httpd restart
/etc/init.d/proftpd restart
/etc/init.d/exim restart
/etc/init.d/dovecot restart

1597 - Reseller IP
ZZZZ - License ID
111.111.111.111 - Old IP
222.222.222.222 - new IP


Source: http://help.directadmin.com/item.php?id=250