Use google, there's a modded G20CI bios made for allowing G20CB to use 7th gen CPU's
Known issues:
1. OnBoard HDMI will not work (G20CI does not have onboard HDMI)
2. LEDs will not work (the LED controllers have another I²C Address etc)
The performance difference between 6700k and 7700k is irrelevant, why would you want to do that?