I would suggest following steps:
1.> Bring down the service group:
/opt/VRTSvcs/bin/hagrp -offline (service_group) -sys (system_name)
2.> Modify the IP with the new one:
/opt/VRTSvcs/bin/hares -modify (IP_resource_name) Address (IP Address)
3.> Bring up the service group:
/opt/VRTSvcs/bin/hagrp -online (service_group) -sys (system_name)
Please ensure that no other clients are accessing the NFS server when you are performing above commands. It would not take much time.
Regards,
Dev.