Well, it's hard to say for sure since this is still pretty early in development but with any early access game it's also best to just accept that you will need to restart worlds a few times as major updates hit. Some change in ways were you can keep a legacy server but it simply won't have everything new available on it, or at least not available in already defined (explored) areas of that map. Others change in ways were legacy saves simply no longer function at all and you are forced to start new whether you want to or not. So far the updates we've gotten are that first category where you can keep your legacy server but may be lacking content in it, but there's no guarantee it will stay that way through all updates up until full release.