Raid 5 migration to bigger HDDs

Yes, that should be possible but:

  1. Have backups… never be without backups (No a RAID is NOT a backup)
  2. If you are running a static volume you only have to grow the static volume, but…
  3. If you are running storage pools, increasing the storage would only increase your pool space and not automatically your volume (people often complain/get confused about it)