Subject to change is one thing. That can be something as little as tweaking numbers, introducing new abilities, adding new content. But what Blizzard actually did over the years is remove content. If you want to play the original Naxxramas, Onyxia's Lair or Zul'gurub, they are not there anymore. If you want to play the original Deadmines or Shadowfang Keep, they are not there anymore. If you want to play through the original zones, they are not there anymore.