The following script will do the needful:
for i in `cat /etc/trueuserdomains | cut -d: -f1`
/usr/local/cpanel/bin/swapip OLDIP NEWIP $i;
OLDIP is the current IP assigned to the domain.
NEWIP is the new IP which you would like to assign.
$i is the domain names read per line from the /etc/trueuserdomains file.