New worlds will have more mistlands area, but you should still find heaps on an old world. Any mistlands you've been in/near may not spawn all the new stuff, though - if you've explored the outer quarter to third of the world it might not be viable.

I haven't played mistlands on an old world myself. There's also a mod that might be able to generate new content in explored areas, I've never used it and it could go wrong - at the very least don't expect any buildings in those areas to survive.

I think your world is also pre hearth & home, which means you've missed tar pits in the plains, and frost caves in the mountains. If you have plenty of those biomes left unexplored you should still be able to find new content.

H&H also brought a food rebalance, health and stamina probably aren't how you remember. And I think meat was just meat, now there's boar, deer, wolf, etc meat - I think old meat just becomes boar meat. The cooking stations have several upgrades now, with various things locked at different tiers. And there was significant changes to fishing, there's various fish and if you want raw fish you have to break them down at a cauldron. Items from Haldor are also starting to be locked behind progression, and mistlands brings at least one new Haldor item locked behind Yagluths' defeat. "Yagluth thing" also now has a name, icon, and purpose.

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Personally I preference starting from scratch. Continuing is not a bad idea per se, but you won't gain much insight into the current state of the early game.

Of course you can always do both: you can have multiple characters and worlds.