As I understand it:

Your old world (world generated before the new content) will not have the new resources as they did not exist when the world was generated. However, you will still get the new recipes and you will be able to import the new resources from another world generated after the update.

If you build something in the mistlands, it will be gone when they release the new biome as that biome will be completely overwritten in the upgrade.

As for the seeds, I have no idea if seed XYZ from before and the new XYZ will be the same landmass layout as before.

Arach