If you are starting a new character I'd say go ahead and start now so you can get some basic gear and recipes, but don't go overboard on the base building. Then when Mistlands drops, start a new map and migrate your character and your stuff over. Just log out of one map and in to the other, making multiple trips and filling chests as you do. Map generation is going to change with the update because the space they previously left for mistlands was too small to generate properly.