Setting up iSCSI to work with ESX is usually a pretty straight forward affair. But like with many things, change can be hard. But sometimes things get moved to different subnets or storage gets replaced. To configure a vSphere client to connect, select a virtual machine and then click on it and click on the Configuration tab. From there, click on Storage Adapters using the Hardware panel.
From the Hardware Panel, click on an initiator and then click on Properties and then click on Configure. Then provide the new name or IP. Make sure that the name is unique and then if needed provide an iSCSI alias. Then change the IP settings if needed and click on save. New iSCSI sessions can be used immediately whereas old sessions will require you to logout and then log back in.