After the DNS records are changed, when users visit your application, the traffic is directed to FortiWeb Cloud instead of your back-end servers. Users will not be aware of the IP addresses of your back-end servers, but the IP addresses may exist in historical DNS lookups that were archived before you activated FortiWeb Cloud service. This could allow an attacker to bypass FortiWeb Cloud and attack your network infrastructure directly.
Therefore, it's recommended to change the IP addresses of your origin servers. Once they are changed, remember to update the IP address in Network > Servers so that FortiWeb Cloud can correctly forward traffic to the new address.