RN104 Disks upgrade from 16 TB (unformatted) to 40 TB (unformatted).

As Marc_V suggests, you can replace one disk at a time (waiting for each resync to complete before replacing the next).  The advantage of that method is that you'll continue to have access to all your data during the process.   The disadvantage is that the process will be very slow (days to weeks), and the NAS will be sluggish while it's proceeding.  Note that the volume will begin expanding when you add the second disk (it doesn't all happen at the end).

But it will be much faster overall if you

1. back up the data
2. power down the disks
3. insert all the new disks
4. power up the NAS
5. reconfigure the NAS and restore the data from the backup.

The disadvantage is that the data won't be available for a few days.

Either way, it's important to make a backup before you expand.  Your data isn't protected during the resyncs in the first method, and if there is a disk failure during that process you will lose all your data.