Netgear ReadyNAS 4312V2 - Replacing older hard disks with new and larger hard disks

Given you have a 12-bay unit that's only half filled, there is a way without SSH.  But it does affect apps and home folders.  Do you use either?

The method would be to turn off XRAID, then use however many larger drives you need to create a second volume, then copy the contents of the old volume to the new and  EXPORT the old volume (EXPORT allows you to put the volume back in, where DESTROY does not).   But personal folders and apps will need some fixing via SSH (see How-to-save-your-apps-when-destroying-your-main-volume-OS6 ).  Backup jobs or SSH are most efficient in copying the files.  I am unsure whether or not you'd be able to turn XRAID back on after that.  There is no technical reason you couldn't, but the OS does prevent it under some conditions.

I suggest you re-think the 4TB drive size.  Larger drives are more cost-effective per TB and make future expansion easier and cheaper.  4x8TB in RAID6 would give you 16TB and better protection from drive failure.  Then just add another 8TB drive whenever you need another 8TB.