Today, Square Enix explained the edits: "The heads and limbs of soldiers under attack by heavy weapons on the battlefield are amended." So probably toned down violence—you won't be able to blow off body parts, I guess.
Square Enix pointed out that the action will be the same and that this alteration doesn't apply to the zombies in zombie mode. So shoot their heads off all you want.
コール オブ デューティ ブラックオプスII 「海外版との仕様の違いについて」 [Square Enix]