The biomes will always be where instructed to by the world seed, and the mistlands biome regions (and all other biomes, including the unfinished ones) have always existed in the game. I'm not certain how it works, but the important thing is that areas you've explored previously may not be updated with new content.

I'm still playing on my old world since almost the beginning of early access. It's working entirely fine. Only issue I've had is having to go slightly further out to discover frost caves (which were introduced in an update this year), since I had already explored the nearby mountains. I had also explored some mistlands coastlines, but I have not revisited them since. The mistlands update visible changed the coastline though, so I'd imagine they could still work. Maybe I'll have to travel back there and see for myself how it turned out.