Agentless Monitor policies (such as those using ICMP Availability Status) rely on the IP address rather than the agents of the targeted resources. When the IP address is changed on a computer the agentless polices will no longer work on that computer. In addition, even if the computer is managed with an agent that automatically reports the new IP address via basic inventory, the agentless policy will still not work because it uses the Network Discovery tables (such as Inv_Device_TCPIP) rather than the basic inventory tables (such as Inv_AeX_AC_TCPIP).
Run Network Discovery tasks on the new IP addresses. Keep in mind that Network Discovery may create Computer resources or more generic Network Resources (which can be viewed in Manage > All Resources > Asset > Network Resource) depending on OS and technologies configured in the Default Connection Profile (such as SNMP). Also Network Discovery may create new duplicate resources so the duplicate resources may have to be manually merged.