When the “Poisonous Snake Token” is destroyed as a result of battle, it is the opponent of “Cobra Jar”’s controller that takes the damage; so even if the opponent had taken control of the “Poisonous Snake Token” with “Change of Heart”, he/she still takes the damage when it is destroyed.
Netrep Rulings:
If “Cobra Jar” is flipped while under the control of the other player through “Change of Heart,” the Snake Token produced by the effect will remain under the control of the player who flipped it.
If the Snake Token is taken control of via “Change of Heart” and destroyed, the opponent of the player who ‘created’ the Snake Token by the effect of “Cobra Jar” will take the Life Point damage.