I still play on my original seed. i exposed about four Mistlands biomes before the update so they remain in their originally cobwebby state. But I've explored multiple updated Mistlands and there are whole chunks of maps where more Mistlands await. I have also exposed a couple of Ashlands and Far North biomes so those will never update, But there is still a ton of them left to discover. My strategy is to not to discover too many of the biomes that haven't been updated yet. I have lots of landmasses left with mountains that I'm sure have crystal caves in them, plains with tar pits, etc. I steer clear of the north and south poles.