Your old save will have Mistlands generated with the old map generation system and will most likely be wonky.
There are mods that "upgrade" your world, but keep your builds and there is a mod to straight up copy paste your builds, but i haven't used them so don't know the names.

Simple search would answer your question.

It depends on how much of the map you explored. New areas only generate
correctly in unexplored areas. My Mistlands start to appear about 1/2 to 2/3 away from
the center. The first Mistlands may be small islands; going furthur you should find larger
areas. good luck

Originally posted by slofish_00:

It depends on how much of the map you explored. New areas only generate
correctly in unexplored areas. My Mistlands start to appear about 1/2 to 2/3 away from
the center. The first Mistlands may be small islands; going furthur you should find larger
areas. good luck

Mistlands normaly only start to spawn really far out.