Yes, that can be changed online. You can click on the cluster name in Prism and change it there, or you can run the command: ncli cluster set-external-ip-address external-ip-address=<ip>

You can change that in either CLI or GUI , You dont need to stop your cluster but this will result in failure if any services configured with Cluster Virtual ip Address,

Example: If you have configured remote cluster with Cluster Virtual ip address  then changing this ip result in error then you will have to go and enter the new ip to establish the connectivity 

Refer this doc for more information

https://portal.nutanix.com/page/documents/details?targetId=Web_Console_Guide-Acr_v4_6%3Awc_cluster_details_modify_wc_t.html