One of the problems with changing the world in Cataclysm to bring all of the classic storylines up to date is that you suddenly go to Outland for an event that happened in the past. You also follow this up with going to Northrend and fighting back the undead which were already shown to have fallen back in the content before you ever went to Outland.
Chris Metzen discussed the fact that this really doesn't mesh well and the internal Blizzard lore folks aren't really happy with it. They plan to eventually go back and make Outland and Northrend fit with the revised storyline. The problem they ran into is purely a lack of time. As an example, Wrath of the Lich King had around 1000 quests in Northrend. Cataclysm has 3500 quests without touching either Outland or Northrend.
So, these changes to make the lore line up are planned, but there is absolutely no schedule as to when they will be implemented.
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