I don't know why the cosmology was changed; I've never been on staff at either TSR or Wizards, and that was an internal design decision. I can only speculate. One thing I have heard is that it covered for "getting rid of" problematic plane names like "Happy Hunting Grounds."
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Mapscientist
@Mapscientist1
Replying to @TheEdVerse
Hey Ed, I had a question I couldn't find an answer to. I've been trying to understand the timeline of the changes to FR cosmology, but and a few things struck me as odd. I was thinking about the implications of the switches between the Great Wheel and the World Tree mostly...