This is called a known plain text attack. Most modern algorithms are hardened against this. So this technique wouldn't help unless the algorithm had a known weakness to this. But then again, maybe people still encrypt their archives using enigma machines these days. The world is a crazy place.

This doesn't apply to ZFS due to the way it uses drives. All drives are added to a storage pool, and drives are used as needed based on speed and reliability requirements. So to upgrade, you'd just add a new drive to the pool, mark the old drive for removal, wait as it moves the blocks to any other drive(s) in the pool, then remove the old drive.