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.