As always, devs have said that new worlds are better for the procedural generation, in experiencing the content as was designed. While old worlds and their mistland areas can be converted and added to, fresh generation was recommended as results might not be the best. Plus, converted areas only works for unexplored regions, and explored regions of mistlands prior to the update, will not convert with the appropriate dungeons and POIs, and likely remain empty. People have also complained about converted terrain generation being unfriendly for sailing and landing ashore, but I'm not sure proper generated terrain is much better, hehe, that might just be the way the biome is and is meant to be.

As to replaying the whole world with a new character to fight all the bosses prior, that is up to you of course, though how soon you experience the Mistlands content may be dependent on how fast you can progress through the other biomes. Personally, I started a new run for a new character on a new world seed, like with all the major updates. Playing Valheim over and over is part of the fun, that it can be grindy, but relaxing, curiously interesting, and beautiful. But others may just get bored at having to grind or repeat stuff again, and so just want to get straight to the new content with a geared up old character. And so it depends on what you'd like to do.