Not really, but I would recommend it. I'm poking at the content with an old character/save, but I'll eventually go all-new as there are a few strange things happening.

You technically can continue on an old world, but there are reports that the biome is not being generated properly. Starting a new world has no problem with it. So you'll either want a new world, or wait and hope that a patch is released that will fix the problem.

The Mistlands only appear in unexplored areas of your map, so if you have most of it mapped out, they recommend you start a new save.