It will probably be like the H&H update. Explored areas won't update but unexplored will. Although I wouldn't be surprised with a new biome added everyone will have to restart their world. No one should complain about it since they signed up for an EA game but they will anyway.

We are most likely 4-6 months from any huge update so i would not worry about it but when they do release Mistlands and the Dungeon updates, i would assume your old worlds will not be compatible with the new update.

We will most likely need new worlds since they need to be able to update the spawn area to compensate for a new trophy stone. At least, this is what I reckon will happen.