Don If the disks in your retired PS6000xv are of the same size and speed as the "under warranty" array, you should have no problem using the disks as cold spares. I would definitely recommend resetting the old one prior to using the disks though.
Is it possible to re-use good disks from a retired PS6000XV on an identical PS6000XV that is still running but out of warranty? Would the retired PS6000XV need to be re-initialized first before trying to re-use the drives or does it really matter? Would this work at all?  I asked EQL support and they did not know. Currently I have 5x PS6000XV's that are still under warranty for a few more months. I am planning on replacing them all, but I may have to run a couple of them past the warranty period before all the new ones can get brought up. Just want to make sure I am covered with plenty of spare disks before even considering this as an option.
Re-Use disks from a retied PS6000XV in a out of service identical PS6000XV