I’ve read/heard somewhere that, even though it’s not necessary to start a new world, devs encourage players to do it to fully experience the new content.
It was the same with some of previous updates (e.g. abominations, frost caves). They loaded into your old world, but fighting them with fully upgraded endgame armor and weapons and high skill levels was not fun at all. At least for me. My primary weapon skill at 90 and jumping and running at level 100 would be really helpful in mistlands, but it feels like cheating to me. So i will definitely start a new world with a new character.