As I see it, you can do a few things:

1. You can locate one of the new biomes, Ashlands, Mistlands, etc., enter it and see if you can build a simple outpost there. If you cannot build structures there that might indicate the developers intend to flesh out those areas in existing seeds. If a viking can't build there then he can't moan and groan online when all his effort gets trashed by new biome content.

2. You can wait on Hearth and Home. We know onions are being added as a new resource. Where they appear should give a pretty good idea of the methodology the developers use to add new items to existing worlds.

3. You can get your knickers in a twist, as the English say, over something you can't control and have no way of finding out. Except demanding the developers take time they would spend actually developing the game to explain to you what they are doing and how they are doing it.

Personally, I prefer waiting to see what's up with the onions in Hearth & Home. Detest onions. Don't like turnips either but my viking seems to tolerate then pretty well.