Any time I am going to do any disk work, I will do it live. The only time I had to shut the system down to change a disk is when I change an entire raid set out for larger drives. First added 4 larger drives to give me just enough room to remove 8 smaller drives. The last step requires a reboot to restricted state to remove the disk from the ASP. Once done, you can IPL and finish the rest of the work live. I just don't know why one would need to shut down the system before doing disk work?
I have replaced many failed disk, including the load source, all live. With this new Power 7 hardware, I even added a communication controller hot. Got to love modern stuff.