Yeah I tried to continue my old world but it turns out the mistlands that generated on it were pretty bad. The terrain was really hard to traverse and weirdly generated. Had to create a new world and move all my stuff over to have a better mistlands experience.

I assume that most of the complaints about the mistlands terrain in the discussions are from users that continued with their old worlds.

But yes, you can continue your old world but depending on how old it is, it might not be as good as starting a new world.