You will need unexplored areas to unlock all the new items. The devs mention Plains specifically in the most recent video update, but new materials might be spawned in other biomes as well.

No
You may not get the new stuff spawning in where you have already explored though.

I would advise if you care about the leveling and progression that you start over. If you don't care then don't bother and just carry on with whatever.

I have a few worlds going I will "upgrade" with new stuff but to check out new systems being put in I am starting a new world with a new character for the update.

But you won't Have to start a new world and old worlds will still function.

edit add - and yes this has been asked way too many times already. People should really learn to use the search function.

no you just dont have spawned new things in an allready explored areas

also you might wanna check:
https://steamcommunity.com/app/892970/discussions/0/3033726413418097054/

Dev Fireside chat:
https://www.youtube.com/watch?v=BUxrOEVMoUQ