Just a quick report that sysupgrade with keeping config works from 17.01.6 to 18.06.1 on a Buffalo WZR 600DHP. I upgraded one of these that's configured as a wireless bridge only (eth1 is not configured/connected anywhere). Basically it's just a WLAN bridge for an old laser printer.
The sysupgrade (from the UI, "keep configuration" selected) went fine, but on reboot I was confused because the default LED behavior has changed between the 17.01 and 18.06 series kernels/wireless drivers.
In the previous kernels, beginning with Attitude Adjustment that I first used on this box, there was no configuration for the WLAN LEDs in /etc/config/system by default. IIRC the wireless driver handled the LED, and was hardwired to what's now phy<n>tpt behavior, and only on the amber WLAN LED.
So when the router booted back up from the upgrade, I saw the wireless LED off and assumed that something had gone wrong with the sysupgrade.
After some troubleshooting I found that it now needs an explicit configuration entry in /etc/config/system for the wireless LEDs. I chose to use the green one instead of amber in my case. BTW it's also possible to use both, e.g. green for radio on and amber for activity, but I found that too busy.
Anyway here's the /etc/config/system addition that works for me:
config led
option name 'WLAN2.4'
option sysfs 'buffalo:green:band2g'
option default '1'
option trigger 'phy0tpt'
Hope this saves someone some time.