The problem with having the world change is that you are effectively cutting off a number of your customers from content that everyone else got to see and participate in.
And that's a valid concern. Many people, who are paying for the content, feel they shouldn't have to miss out on something cool, just because they happened to be on vacation at the time, or whatever. But its also clear that many of us want living, breathing worlds, and if that means quests get removed over time, and zones change, and we miss out on some of those, than so be it. And those two player demands (not wanting to miss out on content vs wanting a world that can change) are hard to reconcile.
City of Heroes has a time travel feature that lets you replay previous story arcs. I haven't seen the feature in person myself, but I wish that type of feature was more prevalent in the MMO industry. I think it would really help work around this type of issue.
One way you could implement this type of game mechanic, more appropriate for a fantasy game, might be by introducing NPC bards or historians that tell tales in taverns about heroic events from the past. The way the system might work is: the player approaches the bard and asks about an event that occurred in the past, like how a group of heroes (or villains) awakened the Sleeper. Instead of simply listening to the tale, however, the player (and friends) would get to play it out in an instanced replica of that zone the way it used to be (more or less). The zone might even scale to the size and level of your group (it, after all, is just a daydream), to keep it challenging.
Of course, loot complicates things. It wouldn't very immersive if people could bring back fabled drops from what is supposed to be a dream. But maybe another reward system could be created -- playing through instanced versions of previous realms gives you "inspirations" or "insights" or accumulate some kind of point total that can be traded in at some kind of item vendor - perhaps for consumables that give you bonuses in the "real life" zones, or even open up quests that take place in the "real world".
If the reward system was compelling enough (maybe I'm the only one who thinks farming old content for consumables that would allow me to better take on the current raid or instance du jour is a good idea), it could be a fun way to keep old content relevant as the story moves on, zones change, and the world evolves.
I think EQ2, WOW, and most other MMORPGs would be a lot better if they would just rip off City of Heroes more.