The case that I had was a FAS2040c with interal 450GB SAS and two shelves DS4243 2TB, and a FS2040 stand alone (no partner) with one shelf DS4243 1TB.
After fully building up the new system, FAS8020 running 8.2.1, I shutdown the stand alone FAS2040 system, brought it up in maintenance mode, removed disk ownership, offlined current aggr0, hot added the 1 shelf of 1TB disk to the FAS8020, and assigned ownership.
I did NOT get any messages about version mismatch nor did it state that it actually changed anything, it did automatically rename the aggr0 to aggr0(1) which was no big deal. But i never ran into any other issues.
Performed the same sequence on my other FAS2040 system with the exception that I needed to take care to assign the appropriate disk to the appropriate controller; halt systems, boot maintenance mode, offline all aggregates, use the 'disk reassign -s XXXXXX -d XXXXXX' (did last step while attached to old system).
I did need to recreate my CIFS shares, create aliases and SPNs respectively.
As a side note the internal 450GB SAS drives were just used for VMFS so those VMs were storage migrated between the two systems.