Yes, given what you said I figured at the least I would have to rename the 19.07.9 files to restore them, but given that it will be swapped to the non-active Flash I won't be able to do that preemptively, right? I would have to again boot to 19.07.9 in the other Flash in order to have access to its filesystem and be able to rename them, yes?
Also, restoring a backup writes it to the non-active Flash, I presume?
Can be done with obvious risks, but if you have serial or are sure of your WIFI... long and short, don't bother, might bork your good'n.
Backup / restore is relative to your current running version.
There is another way if you have the facility. Make no change to your current system, extract the backup that you took, rename the relevant file in the backup, tar things back up, and restore the backup to your newly flashed version.
Thank you to everyone for the replies and advice. I believe that I have things sorted, but not without a few confusing hiccups having to do with inability to connect at several points.
I'm by no means an expert interpreter, but comparing the 19.07.9 files to the default ones created for the 22.02.2 forced upgrade, it doesn't appear to my eyes that there is anything to carry over or merge from 19.07.9. I wound up having to mess with my NAS's connectivity at the same time. I am still confused about the existence of this mysterious "radio 2" WiFi radio and whether it can be constructively used for anything; it's disabled at the moment with an SSID of "OpenWRT", which I assume was auto-assigned to it.
Don't enable radio2 (aka third radio), it shouldn't be by default and would have been there on 19.07 as well. It's a limited radio chip I believe, designed for radar detection. You'll find various topics here that highlight it can be troublesome so best to not have it enabled at all really. The 2.4 GHz and 5.0 GHz radios will work fine (well the best they can given mwlwifi is no longer actively developed). Do not try and enable WPA3, you'll have a bad time.
I'm a little confused by the statement that you cannot keep settings across the upgrade. My understanding is that some routers are exempt from this -- my TP-Link Archer C7, for instance. Is that because some routers don't yet support DSA and are still using swconfig in 21.02?
Correct. If the device has moved from swconfig > DSA, the settings cannot be preserved. But if the device has not yet been migrated to DSA, it is possible to go from 19.07 > 21.02 while keeping settings. In those cases, it is still a good idea to consider not keeping settings (just in case), but it is still technically supported and possible. When upgrading from versions earlier than 19.07, though, you cannot keep the settings.