If I am not mistaken you are worried about settings in your /etc/network/interfaces
In this case just do:
and correct your settings there. Either you can just change your settings or you can add second IP to it (see eth0:1 - that is another IP added to the server)
iface lo inet loopback
iface eth0 inet static
# dns-* options are implemented by the resolvconf package, if installed
post-up ethtool -K eth0 tx off
iface eth0:1 inet static
You should have in mind that if you have a router the settings in etc/network/interfaces will be settings of your router, in this case you need to change settings on your router not on the server!
Furthermore you will have to change all settings in ISPConfig as described above:
1. Set new main IP in Management=> Server => Settings
2. Add further IP addresses at the same place
3. Correct all domains and
4. Correct all DNS records.
You can do it in ISPConfig by clicking and rewriting or you can download your database from PHPMyAdmin (dump database) fix it on your comp in spreadsheet and import fixed database back.
Hope that helps