You can keep your old world. Mistlands will still be there, in completely unexplored areas.

But...the zones will be smaller and may be harder to navigate. It's a matter of luck how well they work out. If you want to take the chance, you will have better luck finding a large area to the east or west, since large chunks of north and south are taken. The map generation will change with the update.

If you don't have a huge base, then I recommend moving your character and your stuff over to a new map. If you are attached to your base, then you can take a chance on the old map, or you can send a "raiding party" to a fresh map.

Added: If you want to reuse the same seed on a fresh map, the land masses will be the same, but the biomes will shift a little and the bosses will move from what I've seen from looking at the updated map website.