This Knowledgebase article details a known issue related to changing the IP addresses on a Neverfail pair of servers.
Using the nfpktfltr setfilter <IP_address> command causes the Neverfail Packet Filter Driver to filter a different IP address than the existing one.
Using the nfpktfltr getfilter command to display the currently filtered IP address, results in the new IP address being truncated when displayed.
This behavior is caused by the nfpktfltr setfilter <IP_address> command providing insufficient buffer space to store the new IP address. However, after using the nfpktfltr setfilter <IP_address> command, even though the new IP address appears truncated when displayed using the getfilter command, Heartbeat has set the new address correctly.
In order to have the nfpktfltr command display the correct new IP address, Neverfail Heartbeat must be stopped and restarted.