If you remove Monster Cards from your Graveyard to increase the ATK of “Bazoo the Soul-Eater”, then on your opponent’s turn he uses “Change of Heart” to gain control of the same “Bazoo the Soul-Eater”, your opponent can remove cards from his Graveyard to further increase the ATK. The ATK increase you caused will end during that End Phase, and the ATK increase your opponent caused will end during your End Phase.
Removing Monster Cards from the Graveyard is a cost of “Bazoo the Soul-Eater”’s effect, so if your opponent chains to the activation of this effect with “Call of the Haunted”, to Special Summon “Kycoo the Ghost Destroyer”, the effect of “Bazoo the Soul-Eater” is not negated since the Monster Cards were already removed.