I am really exited about the update and I cant wait till it comes out! But I have a world that I have worked on for a LONG time and I want to update to take affect to that world. Will that happen?
1.3.1 Only changes some few things like: Inventory in creative, bug fixes.. and SO much more. Its the biggest update yet. But it wont harm your world in minecraft. On my words.
No, but if you want the new feature of the world Genoration alike CoaCoa beans on Jungle Trees, you may want to start a new word, no worlds will be affected.