Areas you have not cleared got updated. If you went there and looked, it won't. If you somehow explored all the mistlands in your map, then its doomed. If you left 5 or 6 chunks, its enough.

You can find Mistlands in old saves, but be aware that the Mistlands are only properly generated in unexplored areas.

Thanks for your answers and comments! It would really help me.

It'll be fine. Prior to the mistlands update I dropped a portal at the biggest spot of mistlands I could see (don't do this). It wrecked a fair sized area and locked in the old cobwebs and stuff, but a little bit beyond the visual horizon it started to generate properly.

I have a habit of wanting to be overly prepared for new content, I also dropped a portal at an Ashland area so I'm expecting the same degree of wrecked land there. Best practice may be to spot an area you want to explore later via map-generation, but don't proceed into any unfinished biomes if you can help it. Playing on expendable maps for this would be the best way.

Personally I think it's kinda neat to have old relic lands merged with the new stuff, gives it a bit of uniqueness and there is NO mist generated in the old world mistlands. So it kinda gives you an advantage for hunting rabbits and fishing/farming. But it won't generate any mines, POIs ect. Not until you see mist.