difference between the ReadyNAS models?

Of course, if you are moving your data to a newer NAS, you can copy the data over to it (backup jobs using NFS first and then changing to rsysnc to verify is the best way) and then update your NV with larger drives to serve as a backup. You would then again set up backup jobs to copy the data back onto the NV, using NFS first and then rsync to verify. You can then leave the rsync jobs set up and set the schedule so your backup job runs automatically. (My jobs start at midnight).

I do recognize that adding a second NAS to the system is a big financial undertaking, especially if we are talking about putting new drives in the new unit and upgrading the drives in the existing unit. I went through that a little under two years ago and thankfully, at that time the price of drives was much less than they are currently. It was a big undertaking for me over several months, but I would not want to go back. The realization when I update my firmware, or have a drive fail, that just in case something goes wrong with one unit, all my data is secure on the other unit is very comforting.