When you create a new volume , the old one will be what I call the "primary volume" (I've never seen an "official" name for it), still containing all apps and Home Folders. When you destroy that primary volume, the other has to become primary. But, sometimes that seems to go wrong, and there are a couple things that never happen.

In addition, your new volume will have to have a different name, and it's a VERY difficult task via SSH to change that. So if you have mapped drives or share call-outs that include the volume name on the NAS itself or other devices, you'll have to update those.

If you have and use user Home Folders, those will not be transferred. The shares themselves won't even be created until the next time the user logs in, and the files will never be transferred. So, you'll have to deal with those yourself. It's a bit of a Catch-22 since you cannot create home folders on the new volume while the old is still mounted in order to transfer the shares. There are some tricks you can use, so let us know if this affects you (I suspect it does not, so won't go into details unless you ask). One of them you can do as a precaution is to EXPORT instead of destroy the old volume. The new one still becomes the primary, but you can re-install the old one (with power off) and it will then be mounted as secondary on power-up. You cannot re-mount a destroyed volume.

Additionally, apps will likely not be properly transferred, though it appears they are supposed to be and maybe that's been fixed in an OS update since I last experimented with this stuff. See How-to-save-your-apps-when-destroying-your-main-volume-OS6 on how to handle that.

As for the chance it simply won't go well, the fact that you have a backup NAS is good. Make certain it's up to date. Should that occur, you have a recovery method. Since this happened to me twice while I was doing a lot of experimenting with just this sort of thing, I don't know if that was because that was a typical failure rate of the process (2 out of 20ish) or was due to the numerous experiments I was performing confusing the OS.

You may want to consider if it's not actually better in the long run to simply shelve the old drives and start fresh with the new, since you have a backup NAS from which you can copy everything. If you find you've forgotten something, you can just swap out the drives and the old volume will be back just as everything is now.

IMHO, you 316 is still a very viable backup to your Synology. For a home user who has not opened any access to the outside world, I really think any security risks of the outdated OS are minimal.

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