• a) If you built something in Mystlands, it will be erased when the new Biome is out.
  
  
• b) If you built something in Mystlands, only the unaltered terrain will be affected when the new Biome is out.
  
  
• c) Old seeds/worlds, created previously of the release of the new Biome will be stuck and no longer updated.
  
  
• d) Taking c) in consideration, you will only have new Biome if you create a new world.

That's is my main concern now. Is it worth to keep investing in my old seed/world to all of a sudden a new updated renders it obsolete?