New content such as tar pits, mountain caves and Mistlands in the future will not appear in areas that have been already generated. Areas are generated 300-500 meters from a player's location. "Exploremap" and "resetmap" has no effect. In other words: Yes, all new content should appear in newly generated areas.

However, take this info from the FAQ with a grain of salt, the developers don't know this for sure, you may need to create a new world to access all the new content. It worked fine with the last updates (almost nobody had problems), but the Mistlands update is way bigger which could lead to more trouble in that regard. We'll see!