I know that my DS215j to DS218+ wasn't in the doable list by moving disks. But I remember doing that which resulted in updated packages (and probably DSM) being downloaded and then the DS218+ continued to run stuff like I had with those disks in the DFS215j.
I did have a full HB backup to fallback on if it went bad. Plus the I had only recently upgraded the DS215j from 2x3TB to 2x4TB (yeah small beer, but I was mid-business case and was planning to say I didn't need to buy extra drives). So the DS215j could take the couple of weeks stale data on the 2x3TB if it went truly bad.