How in the hell do you retcon the Dark Wanderer? An essential part of Diablo 1's story was your hero jamming Diablo's soulstone into his forehead. It was literally the only part of the entire story that the player couldn't skip. Seriously - everytime you killed Diablo it would play the "your hero jams the soulstone in his forehead" cinematic.

The change, I believe (and I still need to follow up on this) is simply naming the Warrior character as Leoric's son. Which, lore-wise, isn't much of a retcon, just giving the Warrior a specific name and origin.