Wasn't using a Browser. noticed old SSID in Windows 8 "Networks" panel as wired connection linked to AC66.
Anyhow, I've downloaded your "RT-AC66U_3.0.0.4_354.27-BETA1" build, might as well move to this firmware if I'm going through the process again.
But, unless I missed something, the recommended method for clearing NVRAM still hasn't been given? And can this process cause problems if done wrong or out of order with the config reset?
I apologize for seemingly beating this thread issue to death with all the cautious questions but experimenting with this sort of thing isn't trivial for me and I don't want to find myself in more trouble.
Thanks to all who have offered help.
The method used does not matter. What matters most is that you do it AFTER flashing, otherwise you will end up with the default values of the old firmware instead of those from the new firmware. There is no need to reset before flashing, since the new firmware is compatible (unlike if you had been flashing Tomato or DD-WRT).
Simplest method IMHO: Flash the router. When it's done flashing note down your important settings, then press and hold the reset button for about 5 seconds. Release it. Give it 2-3 minutes to reboot, then reconfigure everything manually.