You may have to down grade the BIOS...you'll have to go to the board website and read the CPU support list which should indicate the BIOS required for each chip.
A question I have though is why would you downgrade from a 3600 to a 2700 ?
Outside of a very small number of application exceptions that truly require nothing but cores thrown at them there is no workload where the 2700 is faster. I know this first hand because I moved from a 2700x to a standard 3600...and the 3600 is equal to or faster than the 2700x in everything I've thrown at it...this includes gaming, video work, and photo work.