Huh. New 428 cheaper than increasing 316 storage

Yes. This is a great option.

I would use a scratch disk (must not be from your array) to get the RN428 firmware updated to the same version as what's on the RN428, verify it comes up O.K. after the firmware update (i.e. after the reboot), power down, remove the scratch disk and move your disks across keeping the order the same.

You may wish to get some disks for the RN316 at some point and use that to backup important data from the RN428 and/or to provide additional storage if your RN428 fills up.

Your new RN428 would come with a 5 year limited hardware warranty. The RN428 has a newer generation CPU than the RN316 and the RN428 has a quad-core CPU so it's a good step up in performance as well. The use of ECC RAM is nice as well.

I currently use 6TB disks as well. Personally I don't want to get 8TB disks. I want to have a more significant increase in capacity over what I already have when I move to higher capacity disks. I'd like to get e.g. 12TB (or greater) capacity disks but the price is very steep at the moment. Maybe in a few years or so the price will have come down to a level that I'm happy with.