Once you've changed LUN size on storage side, you need to make you host OS aware about that. I'm not familiar with AIX, but here is CentOS / RHEL 5.5 example (assuming that the LUN is known as /dev/sda device on the host):
echo 1 > /sys/block/sda/device/rescan
It will make the host OS aware about the LUN size changed.
Real life configurations are usually more complicated than that and invoke "volume managers", so you'd need to make extra steps to make your volume manager aware about storage side changes.
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