This probably verges more into general system administration for AIX than many of us are typically familiar with. The forums on https://unix.com/aix might be more help.
From some brief reading, there seems to be a "backupios" command and a "viosbr" command set that look to be useful here. If I found myself in your position, I would try creating a new VIOS, then perform the tasks needed with that (maybe sending via internet rather than courier) and verifying the process with a non-production system first.
I am talking about moving VMs around, not the hypervisor in this case.
I found out another way, which is to mount the new iSCSI volume as part of the pool, then "mirror" the instances, and lastly, break the mirror. Only one mirror is allowed, and only one image is allowed to be active.
FYI - no such thing as a snapshot in VIO Server, unless you mksysb and then recover it - aka regular backups.
I was simply hoping for a way to mirror iSCSI volumes directly, to avoid the very slow progression of mksysb and the alternatives. DNE.