Maybe you will need a new world. I played mistlands on my pre H&H world but I did reset the mistlands biome with the world upgrade mod. The terrain is not much different from mistlands in a new world, it felt like there were a lot more dverger structures (but not more infested mines).
The problem is, mistlands also changed world generation. I tried upgrading to mistlands world gen with the mod but that ruined the terrain inside of my base so it wasn't an option for me.
I just started a new world and transfered all of my stuff (I recommend changing inventory size and weight limit temporarily for this with valheim+ mod).