Originally posted by teamguard:

If you have not yet explored an area classified as Mistlands it will generate that biome when you visit any such area. However, unless something was changed in the recent minor patches, the generated Mistlands area for any old map will be pretty bad, just a bunch of really tiny islands.

Not true, something else needs to be done to be able to play your old map/seed.
And I don't want any mod to play the world we already started.