Dnsmasq failure after upgrade from 15.05 to 17.01 and restore

I recently upgraded a 15.x installation on Netgear WNDR3700v2 to "LEDE Reboot 17.01.4 r3560-79f57e422d / LuCI lede-17.01 branch (git-17.290.79498-d3f0685) "
It went well - until I tried to restore settings from a backup that I had made from the 15.x installation.
While it appeared ok there was a significant issue - dnsmasq failed to start because of user:group problems.
I think the issue was probably that 17.x requires a user:group of dnsmasq but 15.x was using root - so it failed to start because user:group was not there.

In the end I reloaded the firmware and redid the configuration by hand rather than trying to patch things.

That is a known difference between 15.05.x and 17.01.x, if you had kept the configuration over the sysupgrade the upgrading scripts should have fixed this particular issue, but restoring an old config does just that, copying over the old files verbatim.

I tried to follow the steps as documented in
https://openwrt.org/docs/guide-quick-start/sysupgrade.luci
and, having read the warnings about the rare cases that Keep Settings is used, I turned that off.

I added some warnings to that wiki page and made the "keeping settings" page less strict.

Thanks - might help others following on behind.