I just upgraded my OpenWrt router from Attitude Adjustment (12.09) to Barrier Breaker (14.07) on my TP-Link WDR3600. There are many methods of doing this, I chose the GUI way in the web interface LuCi. I downloaded the new firmware, uploaded it via LuCi, it flashed successfully and rebooted. My config files were preserved and all of my devices reconnected successfully. Great!
But wait... no LuCi! Whatever I tried, I couldn't get LuCi to connect. After tearing my hair out for a bit, I discovered what was wrong.