You have a change that requires new external IP addresses.
You are not changing any networking conditions on your internal network.
You are using Symantec Messaging Gateway (SMG), or SMG and Data Loss Prevention (DLP) products internally.
DNS records need to be updated to make the new external IPs work with SMG and DLP.
There should be no changes needed in an SMG installation.
DLP in reflective mode should not need any changes.
If there are routing issues after a change in external IP addresses, verify any routing in firewall or switch products that goes to the SMG or DLP products.