New worlds and old worlds; any new content that revamps a biome's terrain will spawn when you *discover* an area.

If youve already explored an area, any future updates that add to the terrain of that biome will not work.

So if you had already explored a section of Plains, you wouldnt get any tar patches when that patch came out.

Now take the Abomination. Its not terrain, just a monster that spawns; so you can have Abominstions spawn in older worlds once you have the new patch.