820 to 520 Upgrade

[Antonio Fernandez-Vicenti <afvaiv@xxxxxxxxxxxxxx>]

A customer plans to upgrade from 820 to 520.
820 presently runs V5R2.
The 520 will be a brand new machine, thus running V5R3.
Some features from the 820 will be moved to the new 520, BUT NO OLD 
DISKS (some 6 and 8GB's) will be moved to the new machine: 520 will just 
use its own 35GB disks

They've been told that 820 needs to be upgraded to V5R3 BEFORE 
saving/restoring things from 820 to 520.

I searched the Archives and found several posts more or less in that 
same line, e.g.:
   "Then and only then when you are on V5R3 can you save the 820 and 
restore it to the 520."

My question is: what's the reason we cannot just save user profiles, 
programs, files, etc fron V5R2 machine and then restore them to the new 
V5R3 machine?

TIA