It is possible to update from https://www.myopenrouter.com/download/dd-wrt-firmware-netgear-r6200-v2-7...
to current Kong firmware from here: http://www.desipro.de/ddwrt/K3-AC-Arm/dd-wrt.v24-K3_AC_ARM_STD.bin.
However after flashing to Kong the only way to update or downgrade is by trying to flash via webgui which results in soft brick. Then tftp the 7-aug-2014 back then flash the desired build. Just sharing in case anyone is interested.
http://www.desipro.de/ddwrt/K3-AC-Arm/TEST/dd-wrt.K3_R6400V2.chk
Sorry, wrong link. Thought you needed 6400v2. Check this directory. http://www.desipro.de/ddwrt/K3-AC-Arm/TEST/