I have tried re-installing the DJI Go app 2.7.2 and power on remote but still no "rc upgrade required" prompt came up.  also try resetting the remote by holding dow c1+c2+recrod+right scroll and power on but still DJI Go app won't ask for RC firmware upgrade.  Please help.

Have you tried the following on my thread? http://forum.dji.com/thread-48599-1-1.html

This was tough for me too because drivers for the aircraft and the remote controller are different and my updated DJI GO app on iOS did not upgade my RC automatically (bug in the new DJI GO app version for iOS users perhaps).
So I have found this useful link for RC upgrade to 1.6 ver. after you upgrade your Phantom 3 aircraft (hope it works with Phantom 4 as well):
https://dl.djicdn.com/downloads/ ... _FW_V01.06.0000.bin

Download it to the root folder of your USB flash then while aircraft and RC are off insert the USB flash to RC’s USB port then turn your RC on and upgrade will begin. Wait for several seconds while LED indicator turns blue and beeps then red then it goes green. Turn the RC off and back on, turn your upgraded Aircraft on and you are ready to fly again with encrypted signal for safer flights.

Fly safe and enjoy good moments with DJI, folks ;)

Origo777