First step, create a new disk on storage and assign it to AIX LPAR

On the LPAR, rescan to find the new disk

[root@aixlpar1]/root # cfgmgr 
[root@aixlpar1]/root # lspv
hdisk4          00fa343cec7cc5e8                    rootvg          active
hdisk5          none                                None

Now mount the NFS share where are located mksysb:

[root@aixlpar1]/root # mount nimsrv:/export/mksysb /mnt
[root@aixlpar1]/root # ls -l /mnt/*aixlpar1*
-rw-r--r--    1 root     system   5787961095 Apr 26 04:38 aixlpar1.mksysb

[root@aixlpar1]/root # alt_disk_install -d '/mnt/aixlpar1.mksysb' -P'all' hdisk5

Now reboot on the new disk image:

[root@aixlpar1]/root # shutdown -Fr

After reboot:

[root@aixlpar1]/root # lspv
hdisk4          00fa343cec7cc5e8                    old_rootvg          
hdisk5          00f732c3ff4939d6                    rootvg          active