Since some people are giving incorrect answers as fact, as per the Existing saves and You thread pinned at the top of the forums, directly from one of the developers:

"We have seen a lot of questions regarding whether or not you'll have to create a new world to get the Hearth & Home-content, and the answer is that it depends. For the vast majority of the content you won't have to create a new server, but some content won't be available if you have explored the whole map. So if you have a lot of unexplored areas there shouldn't be any issues, but if you have explored everything you could miss out on a few things.

We are looking into solutions for this, and with some luck it won't be an issue at all, but for that we can't make any promises. More information on this will come when the update is released!"