no

No, but you'll have to go find undiscovered mistlands biomes to see the new content.

Originally posted by Splatty:

I want to get back in but there's no point in progressing if it'll just wipe my save when Mystland drops.

No but the likelihood you'll be making a need seed post update is high given the beta's suggestion to do so to have more effective mistland biomes on a map.

Just then think of a new seed of such(or cloning yours post release) as a portal world.