Keq does not ALWAYS decrease with increased temperature.
Keq DOES always change with varying temperature, but not in the way you described. Remember, Keq is a constant referring to the equilibrium of a reaction. So technically changing the temperature will increase one direction and decrease the other. It is up to you to determine which way the reaction proceeds. This can be done by determining the delta G of the reaction. The negative delta G would indicate the spontaneous exothermic reaction.
Now back to your original question... Adding heat (reactant) will favor the endothermic reaction direction. Since we found which way is endothermic and exothermic by the delta G's, we can see which direction Keq will increase and which will decrease.