ReadyNAS 312 and 422

Personally I'd set up the RN422 from scratch, and then use the built-in backup jobs to migrate the files.  It will take about the same amount of time (perhaps even less), and gives you a completely clean file system on the new NAS.  The only thing you'd lose would be the snapshots - which for me are nice to have, but not worth migrating.

But the procedure you sketched out can work. Adding more detail, it looks like this:

1. Update the RN312 to 6.9.3 (if you aren't running it already).
2. Power down the RN312 and the RN422
3. remove one disk from the RN312 and insert it into the RN422
4. Power up the RN422, and change the hostname.  If you have configured a static address, then change that too.
5. Hot insert the 3 TB disk, and wait for resync to complete
6. Hot-swap the 2 TB disk (replacing with the second 3 TB drive), and what for resync to complete
7. Zero the 2 TB disk.  You can do that in a Windows system, using vendor tools (Seatools for Seagate, Lifeguard for Western Digital).  
8. Power up the RN312
9. Hot-insert the zeroed disk, and wait for resync.

I do suggest making a backup first.

On step 7 - if you don't do this, you will run into issues when you reach step 9.  Selecting the reinserted disk and clicking on "format" might be enough to get past them. But I think zeroing (or un-formatting) the disk before you reinsert it is the safest path.

There might be some other adjustments to apps and cloud services needed in step 4 (or on the RN312 after step 1).  For instance, if you want to stream over the internet from the plex server on both machines, you will need to adjust the server ports.  Similarly if you use ftp or ftps for remote access - the control port and the passive ports will need to be set differently on the two NAS.