The new stuff will spawn 500 meter into unexplored areas. Will not apply to already explored areas.

There is a mod that can update explored plains to have tar pits. Caution if you have built in plains.

Bonus tip. There is a command you can use to update terrain to new better performance terrain within a short vicinity. Caution again if you have stuff built at max and min height troubles are due. I can't recall what the command was, but it's out there.