Minecraft] How do you get a 1.16.3 world into the 1.17?

There will be new mobs, biomes and items. But will the not yet generated areas be able to take the 1.17 features if these areas have not yet been loaded?

Because there won't be any of these things in the area and you can walk around forever to find copper, for example, or something…

As far as I know, only the "new" chunks (that is, chunks that have never been loaded) will have the new chucks, etc., at least that was the case with all other changes to the terrain creation

If you then open the world in 1.17, only the chunks that have never been generated will have the new features.

Edit: It was once possible to delete old chunks and have them regenerated.

Yes, I think you can delete chunks with MCEdit up to 1.11