In H&H, tar pits would not spawn in plains that was already discovered, but would in plains yet to be explored.
I heard, cant remember where, that the current plan for mistlands is not to change any mistland biome already discovered...

You should be able to keep an old seed and still have the new contents, but only in areas not yet explored, probably even be able to use the same seed to re-create the world (but you would need to explore and re-build again, but if you enjoyed that seed it will give you a heads up).