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The only reason anyone has seen the original world again is because of black market coders
A Legacy World Portal in each starting zone.
Travel between Legacy Worlds at any time in any order on one character and server.
Walk into the Legacy Portal and select the Legacy World:
Cataclysm Azaroth (80-85)
Broken Isles Azaroth (100-110)
You die and drop your body, abilities and items on one side of the portal,
Only your soul can transgress the portal.
You would retrieve a new body in the starting zone at the lowest level appropriate for that world.
Equipped with only the most basic starting gear and abilities.
You would retrieve your previous body at the level you had last progressed in that world,
Equipped with all the items and abilities you had acquired there.
Apply as they were independently in each Legacy World.