Last time I did this was with some R77.30 but believe that R80.x would be the same but haven't had to readdress and R80.x Gateway/Cluster.
1.) Make sure that routing on the Network is in place to get to the New Interfaces correctly, this is VERY important
2.) Simply change the Cluster IP in the SmartConsole, Check VPN Link Selection settings etc to make sure not affected.
3.) Change the Cluster Member IP addresses
4.) Install Security Policy
5.) On the Standby Member then use the WebUI to set the Management Interface as Mgmt. This will update the localhost entry so that resolves the address of the cluster member to be the Mgmt Interface IP.
6.) Reboot the Standby Member
7.) Failover the Cluster to the Standby Member
8.) Repeat Step 5 and 6 on the Member that is NOW the Standby
9.) Failover the Cluster to the Standby Member so that the original active member is active again
This is what did and worked fine for me without breaking the cluster