Modders have already made tools to remove and recreate outdated regions of worlds, so no.

My fort seed is an original RNG roll that's been played the whole time, no issues yet. 630 in game days, and purchased last feb.
GL!
I am meters away from a 500+day old corpse, and the raft has been bobbing on the shore for two in game years almost. Eerie as heck. Like stepping into a Fri13th movie.

I think the world will generate the new biome if you have not explored all the map. You dont have to restart youe character either. I suggest to keep playing with out fear. The game is still on Early Acces but it's still really big so we hace to keep learning.

I've had the game for about a year or more now, and my original characters from when I bought the game (well before Hearth and Home), are still playable. I expect at the very least our saves will be useable until a full official release (and even then likely possible).

As for new biomes, I'm not sure how that will work. But currently there are 3 biomes already in the game (so they are spawned in the map), but they are essentially empty. They have their decorations and such, but no active monsters or anything. I imagine any content updates that deal with these biomes will just add more events/ai and such to the already spawned biomes.

For example, you can end up finding the Mistlands biome, but there's nothing spawned there. Just the landscape and doodads. It looks really cool, btw! Can't wait till it's actually populated with monsters.

Unexplored areas beyond 500 feet from explored areas will generate new biome terrain-related content when you are within sight of them.

New content that aren't biome terrain related, but appear only in specific types of biome like the Abomination have so far, been able to generate in explored areas.

For example, I surmise Mountain caves will not generate on explored mountains, because they are biome terrain specific. However, if IronGate introduces goats into mountains, the animals will generate on explored areas because they are not a part of the terrain, but entities on their own. Make sense?

On my dedicated server, we have a loose-rule, where players can explore beyond the islands directly around spawn, if all the areas around spawn have already been claimed enough. Therefore, there are still a lot of areas that are left to explore. However, I surmise by the time two biome contents are added to this game, we might just start a new server world.