Originally posted by Flare |UKCS|:

The last major update , Hearth & Home did not break worlds, but the new tar pits would not spawn in plains areas you had already explored.
The Mistland stuff will probably be the same, only change areas the players has not already explored.

To be honest though I just restart the game personally to fully enjoy the new stuff, but thats me.

Yeah, they try their hardest to keep old worlds compatible. Occasionally they can break them, especially heavily explored worlds.