Upgrade of drives due to low space

The drives are Linux formatted, so your PC or Mac is not going to be able to read the drives that were put in the expansion.  If you make sure the volume name of the volume on the new drives doesn't match the one on the old, you might be able to mount the volume on the NAS itself.  You could also mount them from a Linux bootable system.  But unless you have Linux experience, these are probably not your best plan.  There is software you can run under Windows that'll do it.  But it's designed for recovery and quite expensive.  Since your volume is intact, it sems like a needless expense, better spent on a backup means.

Your assumptions on RAID are very wrong.  You cannot think of a RAID array as individual drives.  The data is spread across them as if they were one continuous larger drive, plus parity information for redundancy.  That's why you need the whole set.

Unless you have a sudden need for that much extra space, you are better off only swapping two drives now, and the others as you need more.  Besides the price likely dropping, you spread out the manufacture dates and operational hours of the drives, making simultaneous failure later less likely.

Anything you see regarding the space limits of an OS6 based ReadyNAS are simply based on the largest drives available at the time of release.  The 214 will handle 10TB drives just fine, and should handle 100TB if they ever become available.  Of course, with the limitations of the ARM processor and memory, four 100TB drives may not be practical.