Is it possible to migrate?

It would be a very long process, as each step would require a re-sync.  You'd first need to switch from RAID6 to RAID5 using this process:

https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/RAID6-to-RAID5-without-volume-re-creation-is-possible/td-p/1928861 .

Then swap out three of the drives, one at a time and allowing for re-sync, using normal drive upgrade process.

Lastly, you'd have to remove the remaining 4TB drives using this process:

https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/Reducing-RAID-size-removing-drives-WITHOUT-DATA-LOSS-is-possible/td-p/1736125 .

I think you could do that last step all at once (removing all the unwanted drives with one command/re-sync), but I've not tried it.

And you'd need to insure you have a backup before starting, as there are a lot of steps where something can go wrong.  IMHO, not worth the effort.  Just make the backup, swap all the drives, re-create the environment (restoring a saved configuration file will go a long way in accomplishing that), and then restore the files.  If you have apps, make sure you install them before restoring the configuration backup.